Here's the list of useful FREE Application for your Mac OS X. List will be updated from time to time to make sure that every applications on the list are totally FREE.

Business Applications
Adobe Reader - Read PDF files using this application.
Skim - a PDF reader and note-taker for OS X. It is designed to help you read and annotate scientific papers in PDF, but is also great for viewing any PDF file.
ThunderBird - a free email application that's easy to set up and customize - and it's loaded with great features!

Utilities
AppCleaner - a cutting edge software that allows you to safely uninstall apps while keeping the registry intact.
Burn - An application used to burn discs that offers a lot of advanced options.
Caffeine - a tiny program that puts an icon in the right side of your menu bar. Click it to prevent your Mac from automatically going to sleep, dimming the screen or starting screen savers.
Chicken Of The VNC - a VNC client for Mac OS X. A VNC client allows one to display and interact with a remote computer screen. In other words, you can use Chicken of the VNC to interact with a remote computer as though it's right next to you.
ClamXav - a free virus scanner for Mac OS X. It uses the very popular ClamAV open source antivirus engine as a back end and has the ability to detect both Windows and Mac threats.
Deeper - a personalization utility for OS X which allows you to enable and disable the hidden functions of the Finder, Dock, QuickTime, Safari, iTunes, login window, Spotlight, and many of Apple’s applications, and more.
F.lux - makes your computer screen look like the room you're in, all the time. When the sun sets, it makes your computer look like your indoor lights. In the morning, it makes things look like sunlight again.
Flame - If you've ever wondered what's going on behind the scenes on your network, then Flame can show you.
GeekTool - an application for OS X 10.6 or later. It lets you display various kinds of information on your desktop via 3 default plug-ins:File plugin,Shell mode, and Image mode.
IP Securitas - the most advanced, yet free IPSec client for Mac OS X. It supports virtually every available IPSec compliant firewall, allowing you to connect safely to your office or home network from any location on earth.
iStumbler - the leading wireless discovery tool for Mac OS X, providing plugins for finding AirPort networks, Bluetooth devices, Bonjour services and Location information with your Mac.
KissMac - an open-source and free WiFi stumbler/scanner application for Mac OS X. It has an advantage over MacStumbler/iStumbler/NetStumbler in that it uses monitor mode and passive scanning.
Loginox - Change your Login background Image with a single Drag-and-Drop.
OnyX - a multifunction utility for OS X which allows you to verify the startup disk and the structure of its system files, to run miscellaneous tasks of system maintenance, to configure some hidden parameters of the Finder, Dock, QuickTime, Safari, Mail, iTunes, login window, Spotlight, and many of Apple’s applications, to delete caches, to remove a certain number of files and folders that may become cumbersome, and more.
Monolingual - a program for removing unnecessary language resources from OS X, in order to reclaim several hundred megabytes of disk space.
PresentYourApps - lets you hide the Dock and menu bar for every Cocoa application you choose.
Remote Desktop Connection - you can connect from your Macintosh computer to a Windows-based computer and work with programs and files on that PC.
Rookit Hunter - removes popular UNIX and Mac rootkits.
Server Admin Tools - This application provides information about the Mac OS X Server administration software applications.
Silverkeeper - a freeware application from LaCie that automates backups to external storage devices like hard drives, network drives, and other removable storage devices.
SMARTReporter - an application that can warn you of some hard disk drive failures before they actually happen! It does so by periodically polling the S.M.A.R.T. status of your hard disk drives. S.M.A.R.T. (Self-Monitoring Analysis and Reporting Technology) is a technology built into most modern hard disk drives that acts as an "early warning system" for pending hard disk drive problems.
Spectacle - Move and resize windows with ease. Window control with simple, customizable keyboard shortcuts.
The Unarchiver - a small and easy to use program that can unarchive many different kinds of archive files. It will open common formats such as Zip, RAR, 7-zip, Tar, Gzip and Bzip2. It will also open many older formats, such as StuffIt, DiskDoubler, LZH, ARJ and ARC.
Transmission - an open source, volunteer-based project. Unlike some BitTorrent clients, Transmission doesn't play games with its users to make money.
µTorrent - an efficient and feature rich BitTorrent client sporting a very small footprint. It was designed to use as little cpu, memory and space as possible while offering all the functionality expected from advanced clients.
Vuze - an end-to-end software application for all your torrent needs. Although it is a complete bittorrent downloader, the Vuze program maintains a lightweight footprint, doesn't slow your computer down, and quickly downloads torrents.
Wake Up Time - a handcrafted, simple to use alarm clock. It has everything you need for using an alarm clock on a daily basis.
WireShark - the world's foremost network protocol analyzer. It lets you see what's happening on your network at a microscopic level. It is the de facto (and often de jure) standard across many industries and educational institutions.

Social Networking
Adium - a free instant messaging application for Mac OS X that can connect to AIM, MSN, Jabber, Yahoo, and more.
Camfrog Video Chat - allows you to join real streaming video chat rooms where you can hear, see, and chat with many people at a time.
Colloquy - an advanced IRC, SILC & ICB client which aims to fill this void.
ICQ - allows you to chat with all of your friends in one app. It’s convenient and free, so you can forget about costly SMS and MMS.
Line - a revolutionary communication app that enables you to enjoy free messages and voice calls anytime, anywhere!
Skype - is for doing things together, whenever you’re apart. Skype’s text, voice and video make it simple to share experiences with the people that matter to you, wherever they are.
Twitter - the place to get breaking news, local updates, global events, inspiration and good conversations.
TweetDeck by Twitter - makes it easier for publishers, marketers and power users to track the real-time conversations they care about. It brings more flexibility and insight to power users through a customizable layout that lets you keep up with the people and topics that matter most to you. And, you can join the conversation by tweeting, sharing photos and links to news stories, and more.

Reference
Mactracker - provides detailed information on every Apple Macintosh computer ever made, including items such as processor speed, memory, optical drives, graphic cards, supported OS versions, and expansion options. Also included is information on Apple mice, keyboards, displays, printers, scanners, speakers, cameras, iPod, Apple TV, iPhone, iPad, Wi-Fi products, Newton, iOS, Mac OS, and OS X versions.

Productivity
Alfred - a keyboard-driven productivity application, helping you launch apps and search your local computer as well as the web with great speed.
Apache OpenOffice - the leading open source office software suite for word processing, spreadsheets, databases, graphics, presentations and more.
Anxiety - Anxiety is a super-lightweight To-do list application for Mac OS X Leopard that synchronizes with iCal and Mail. Its aim is to provide a streamlined, easily accessible interface to add and check off your tasks, while remaining poised to melt into the background at a moments notice.
BackBoard - I love my Mac and my Backpack Account. Both together gives me not only the possibility to write my notes and tasks down, but more important to write them down EASILY!
BootChamp - Quickly boot into Windows using BootChamp
Evernote - Evernote makes it easy to remember things big and small from your notable life using your computer, phone, and the web. Everything you capture is automatically processed, indexed, and made searchable. If you like, you can add tags or organize notes into different notebooks.
Firefox Browser - a popular open-source graphical web browser from Mozilla
Garagebuy - GarageBuy is an app primarily targeted at collectors and power-buyers on eBay. It will keep track of your searches and can also synchronize them to your other iOS devices. It will also synchronize the list of items you already looked at, so you can focus on new items exclusively, no matter if you are on your Mac, iPad or iPhone.
Houdini - a free Mac menu bar utility that hides applications which have been inactive for a certain period of time. Stay focused and clutter-free with Houdini.
Imagine - A simple, clean, readable layout and all you'd ever expect from a simple text editor, but so much more. You no longer need TextEdit, or Word. Imagine can do it all for you, in a faster, more beautiful package.
LibreOffice - the power-packed free, libre and open source personal productivity suite for Windows, Macintosh and GNU/Linux, that gives you six feature-rich applications for all your document production and data processing needs: Writer, Calc, Impress, Draw, Math and Base. Support and documentation is free from our large, dedicated community of users, contributors and developers.
NameChanger - designed for the sole purpose of renaming a list of files.
Namely - possibly the fastest way to open applications.
nvALT - a fork of the original Notational Velocity with some additional features and interface modifications, including MultiMarkdown functionality.Notational Velocity is a way to take notes quickly and effortlessly using just your keyboard.
Ommwriter Dãna I - your own private writing room where you can close the door behind you to focus on your writing in peace. Everywhere you go, you have access to a beautiful distraction-free writing environment where your authentic voice is free to go where it is meant to go.
Quicksilver - A unified, extensible interface for working with applications, contacts, music, and other data.
Skitch - lets you communicate your feedback and important ideas in an instant. Use Skitch to mark up a PDF, snap your screen, annotate a photo, or draw something new. Share your thoughts with friends or co-workers. Annotate images and PDFs with arrows, stamps, text, shapes, and more. Getting everything done just right has never been easier.
Slife - helps you understand how you spend time in the computer. It automatically observes as you interact with applications, documents and web sites.
Snippely - a basic text and code organizational tool. Instead of storing bits of code, quick notes, and memos in text files all over your hard drive, this application will let you save and organize "snippets" in one convenient location.
Task Mate - The to-do list made simple again. Just click to add, remove or complete tasks. Your completed tasks are easily accessible. Export your tasks to other applications via TextEdit.
VirtualBox - a general-purpose full virtualizer for x86 hardware, targeted at server, desktop and embedded use.

Audio
8tracks Radio - light and gorgeous 8tracks application that lives in your Menu Bar and in very few steps you can dive into your favorite mixes.
Audacity - a free, easy-to-use, multi-track audio editor and recorder.
Juice - This program lets you create your own custom online audio anytime, anywhere.
Muzzy for iTunes - a tiny and gorgeous iTunes companion that lives on your Menu Bar. Made to be simple, Muzzy allows you to view your track's artwork along with its information (current time and name) in an effortless way.
VirtualDJ Home - the most widely used DJ software, and the uncontested leader of non-timecode digital DJing. It has been used around the world for the past 10 years by bedroom DJs and professional superstars alike to rock millions of parties, and has been chosen by leading DJ brands such as Pioneer, Numark, Hercules, Denon, Gemini, or Vestax, to be included with most of their digital DJ controllers.
VOX - a simple, yet powerful solution for all your music listening needs.

Video
Blackmagic Disk Speed Test - an easy to use tool to quickly measure and certify your disk performance for working with high quality video! Simply click the start button and Disk Speed Test will write test your disk using large blocks of data, and then display the result.
Centerstage Project - a project to create an open source media center for the Mac platform which is easy to use, customisable, and expandable using plugins.
HandBrake - a tool for converting video from nearly any format to a selection of modern, widely supported codecs.
IPodDisk - a tool designed to make it easy and intuitive to copy music from your iPod.
Miro - An amazing, award-winning open-source, non-profit music and video player and converter.
MPlayerX - multimedia player with the new eye-candy fashion.
VLC - a free and open source cross-platform multimedia player and framework that plays most multimedia files as well as DVD, Audio CD, VCD, and various streaming protocols.

Photography
Fotor Photo Editor - A photo processing engine makes it easy to process high quality photos quickly. Adjust exposure, brightness, contrast, white balance, and saturation, sharpen/blur, add a vignette, or crop, straighten, rotate and more. And new editing tool is Red-Eye Removal. Fix scary pictures fast with our new awesome red eye reduction tool.
GIMP - a Freeware photo editing application.
Inkscape - an Open Source vector graphics editor, with capabilities similar to Illustrator, Freehand, CorelDraw, or Xara X using the W3C standard Scalable Vector Graphics (SVG) file format.
Paparazzi! - a small utility for Mac OS X that makes screenshots of webpages.

Developer Tools
Brackets - an open source code editor for web designers and front-end developers.
Emacs - a text editor of legendary power and configurability, but it also has an enormously complex user interface.
FileZilla - a fast and reliable cross-platform FTP, FTPS and SFTP client with lots of useful features and an intuitive graphical user interface.
MAMP - installs a local server environment in a matter of seconds on your Mac OS X computer, be it MacBook or iMac.
Nvu - a standalone 'WYSIWYG' (WhatYouSeeIsWhatYouGet) HTML editor based on Gecko, Mozilla's rendering engine.
Set Icon - a small application that lets you quickly and easily set a custom icon for one your drives either from an ICNS file or any old image file that Mac OS supports.
SvnX - an open source GUI for most features of the svn client binary. It allows you to browse your working copies, spot changes and operate on them but also to browse logs and revisions of your repositories.
TextWrangler - a powerful and richly featured tool for composing, modifying, and transforming text stored in plain-text files.

Graphics & Design
SketchBook Express - a fun and intuitive drawing application. Get a taste of the SketchBook Pro experience with SketchBook Express. Use professional-grade tools and brushes to create and save flattened images to your Mac.
Games
CSR Racing - Race your dream car in the ultimate test: a drag race along deserted city streets. CSR Racing combines stunning, high-fidelity graphics with addictive gameplay to create a new type of racing experience.
Pocket Planes - From the creators of Tiny Tower comes Pocket Planes, a casual airline sim that fits right in your pocket!
SHADOWGUN: DeadZone - The award-winning multiplayer shooter now available on your Mac computer! Show them what you’re made of in your quest to become the most-wanted bounty hunter in all the galaxy. With your friends at your side, experience larger than life multiplayer battle!
Touchgrind - The innovative finger controls together with true physics simulation let you pull off tricks like ollies, shuvits, kickflips, heelflips, impossibles, 5-0, 50-50, crooks, smithgrinds, boardslides, tailslides, darkslides etc in endless combinations.