Jarannis's list of FREE programs

Windows

Video Card Tweaking
ATITool - Allows you to fiddle with your radeon card.

General System Sensor and Overclocking tools
CPU-Z - Tells you things about your cpu that your bios won't.
Sys Tool - Allows you to overclock almost everything but your case.
PowerStrip - optimise your display and GPU.
ClockGen - Quick way of overclocking your pc through windows.
Sandra 2005 - Probe your system for information, benchmark your components.
Throttle Watch - Detect throttling in your pc.
Speed Fan - Monitor your fans and adjust the voltage applied to them.

Game Session Recording Tools
FRAPS - record videos of your games, benchmark while playing games.

Game Server Bookmarking
HLSW - Bookmark gameservers you like, and find details of said servers.
System Utilities
Pagefile Defragmenter
Office
Open Office
Notepad++ Notepad, but with extra tools and such.
Programmer's Notepad
Win32Pad
AbiWord

Shell Replacements (full)
LiteStep

Theming Programs
Boot Skin - Change the windows welcoming screen for the better.
WindowBlinds - Make your Desktop look all fancy,

Expanded Panels
Object Dock - Scrolling icon dock, similar to the Mac OS X, but with useful applications.

Online Antivirus scanners:
Panda Active Scan
Trendmicro Housecall

AntiVirus
AVGfree (recommended)
avast!

AntiSpyware
Spybot S&D (caution: it is possible to remove important system information with Spybot)
HijackThis (caution: it is possible to remove important system information with HijackThis)
Adaware
Ewido - from GRIsoft (recommended)

Graphics Editors:
Paint.NET
GIMPshop
Inkscape

Internet Relay Chat (IRC)
Xchat2 - Free and Legal Xchat Build.
HydraIRC

Instant Messengers
Trillian Basic
Gaim

Browsers
Mozilla Firefox - Recomended
Opera - Recomended
Maxthon

E-Mail Clients
Thunderbird
Outlook Express - Free with internet Explorer

VoIP/Telephony:
Skype
Gizmo

Game Messaging Services
Xfire

Audio Editing
WavePad
Audacity

Media Players
Media Portal
Winamp
Foobar2000
JetAudio Basic, good Audio Player for windows
xmplay lightweight media player.

Codec Pack
K-Lite

BitTorrent Clients:
BitComet
uTorrent

Multiple Monitor Utilities
Multimon

FTP programs:
FileZilla
SmartFTP

3D Rendering Programs:
Blender


Linux:

Web Browsers:
Mozilla Firefox
Flock
Opera

Email Clients:
Mozilla Thunderbird

Graphics Editors:
The GIMP
Inkscape

Instant Messaging:
Gaim

VoIP/Telephony:
Skype
Gizmo

Compilers:
GCC/G++

Productivity Suites
Open Office

Music Players/Downloaders:
amaroK

BitTorrent Clients:
BitTornado


Mac OSx

Web Browsers
Shiira
Firefox
Safari - Recomended
Flock Browser

E-Mail Clients
Thunderbird
Mail.app - Recomended

Instant Messeging
AdiumX
aMSN

Media Players
VLC Player
Flip4Mac

Anti-Virus
ClamXav

Music Players
iTunes

Graphic Editors
GIMPshop - Recomended
The GIMP

Office
NeoOffice

FTP
Fetch FTP - 15 day trial, with nag screens afterwards.
One Button FTP - Kinda hard to use, and it don't save your FTP password.