This will be a list of free or open source programs or utilities. This is not my list, I take no credit for it as I've compiled it from a bunch of different sites and added some items I've personally tested and found to be useful. Post here if you have anything to add, please provide a link and a brief description of what it does. Note this is for free programs only, this is NOT for posting cracks or pirated software.
Anti-Virus Clients
Avast! Antivirus Free Anti-virus client for home users
AVG Highly rated free Anti-virus app
Spyware Terminator Despite the name it's actually an AV scanner that also picks up spyware, keyloggers, ect.
Microsoft Security Essentials Microsoft's free, lightweight, and very good AV/Malware scanner. Requires you to pass a Genuine Windows test.
Anti-Spyware Clients
Adaware 2008 Good free spyware scanner, no realtime protection but good detection rate
Spybot Search and Destroy Highly rated free Anti-Spyware utility
Super AntiSpyware Another good anti-spyware program
Hi-jack This! Advanced tool used to locate and disable hidden spyware processes or registry entries -use caution when using-
Boclean Real time spyware removal tool
MalwareBytes Good tool for removing lots of spyware/adware/malware
Firewalls
Comodo Personal Firewall I like this firewall a lot, it's great for being free, had adjustable security levels and can "learn" what programs to let through
PC Tools Firewall Plus Free basic software firewall
Alternatives to popular software
Trillian Good all-in-one messenger
Pidgin Used to be GAIM, same purpose as Trillian
Digsby Integrates messaging, social networking sites and e-mail. No IRC functionality.
Firefox Widely popular web browser, has ton of extensions and security options that make it more secure than IE
Thunderbird From makers of Firefox, alternative to Outlook
VLC Media Player Media Player that will play tons of formats
Quicktime Alternative Plays Quicktime files without the bloatware and mem hogging processes
Real Alternative Same as above except for RealMedia files
Filezilla Free ftp client
GIMP Open Source Photoshop alternative, high functionality
Paint.Net Free photo editor with many options
Foxit Reader Like Adobe Reader without the obscene load times or bloatware
Open Office Open Source version of Word, Excel, Powerpoint, ect and will open files created in MS office
CutePDF Free PDF creator
7-zip Like Winzip
System Utilities
Bart PE Creates a bootable disk that allows you access to a command prompt, registry, ect when unable to boot into Windows, has many other uses
CCleaner Good disk and registry cleanup utility
SpeedFan Temperature Monitoring and fan control program
Undelete Plus File Recovery Program
CPU-Z Shows info about your processor, voltage, clocks, ect. Good when testing overclocks.
Keepass Store and manage usernames and passwords
Memtest86 Utility for testing RAM
Prime95 CPU stress test, universal standard for testing system stability when overclocking
Orthos like Prime95 but better support for multiple-core CPUs
3dMark06 Widely accepted standard for benchmarking video cards
Pc Inspector Another file recovery program
Synergy Share your mouse and keyboard across multiple computers, seamless and great if you have multiple computers running next to each other
Crossloop Free Remote assist software
Eraser Fully deletes and destroys file data
LookatLAN Network discovery tool
Media Player Classic builtin codecs similar to VLC, but in the "good old style" of WMP 6.4 and with a few bonus features
GPU-Z does for GPUs what CPU-Z does for processors
Orbit/Grab++ download manager, with its component grab++ you can even download streamed songs and flash-vids from youtube &co
SUPER extremely powerful audio/video converter, has support for special formats (like for consoles or mobile phones)
JDiskReport shows disk usage of folders and files, requires java
FreeCommander one of those countless Norton commander clones (), nice alternative for windows explorer
Rivatuner a very good graphics card utility with tons of features
Defraggler A very popular defragging program
JDiskReport A handy program for finding where all your hard drive space disappeared to; gives a report of the space consumed by folder.
Misc Utilities
Skype Make phone calls from your PC. great for long distance calling
Safari Apple's web browser, works on Windows
Daemon Tools Virtual Drive manager, can mount an ISO instead of wasting CD/DVDs
DVDfab Rips DVDs and copies to your hard drive, and removes protection
Winamp Widely popular music player
CCCP Codec Pack Codec pack that has pretty much everything
Everest Ultimate Shows pretty much everything you'd want to know about your computer hardware
Samurize lets you build individual desktop widgets and monitoring boxes, has tons of user-made layouts available
ATITool monitoring/benchmark/overclocking software for GPUs (yup, even for nvidia ones ^^)
Hamachi LAN parties across the net ^^ uses encrypted VPN tunnels
CSpace decentralized instant messenger, uses TLS encryption and offers file transfers as well as remote desktop functionality over the VNC protocol
FLV Player easy player for viewing those .flv files ripped from video streaming sites
Notepad++ not just a notepad clone, but a full-fledged editor for pretty much any web/programming language (and of course a nice replacement for good old notepad ^^)
Eclipse free development platform, entirely written in java
CD Burner XP CD burning software like Nero
Virtualization
Virtual PC Microsoft's virtualization software
Virtual Box Multi-platform client
VMWare Robust Virtualization software. They charge for some services, but many of the features are now free.
Mac OSX
iStatPro Computer stat widget
Adium Mac messaging client
Colloquy Mac IRC client
Eclipse Coding platform
SMC Fan Control Temp regulating app for Macbooks
Textwrangler Like Notepad++ for windows
Please feel free to continue to add to this list. Thanks!