I put this together for another forum. It may be a little out of date now, there are bound to be duplicates, but here it is:
Security
Antivir An anti-virus program.
Avast An anti-virus program.
Grisoft AVG An anti-virus program
Crap Cleaner CCleaner is a freeware system optimization and privacy tool. It removes unused files from your system - allowing Windows to run faster and freeing up valuable hard disk space. It also cleans traces of your online activities such as your Internet history.
AdAwareSE Malware cleaner
A-Squared Malware cleaner
Note: No longer free as at April 2006. Trial version only.
Spybot Search & Destroy Malware cleaner.
Spywareblaster Malware preventer. Blocks malware. Runs in background.
Video Drivers
Guru 3D All latest Video drivers for every make of video card including Beta drivers. Tools, benchmarks, tweaks, all things concerning video cards.
Omega Drivers Optimised and re-written drivers the author claims improve gaming. For Nvidia & ATI cards.
Nividia Home Page For latest drivers straight from the manufacturer.
ATI Home Page For latest drivers direct from the manufacturer.
Tools
Memtest Tests motherboard memory. Download program or ISO then boot from floppy or CD.
Ultimate Boot CD Consolidates as many diagnostic tools as possible into one bootable CD.
Speedfan Monitors temperatures, voltages and fan speeds.
Motherboard Monitor Monitors temperatures, voltages and fan speeds. Last version made in 2004, suitable for older motherboards.
Bootdisk Download bootdisks. Including the one you need to have to flash a Bios.
Prime 95 A CPU Stress test, ideal for checking if your overclock is stable.
CPUID. Gathers information on your CPU; Memory; Motherboard & system, including FSB & Multiplier.
Bart's bootable live Windows CD/DVD Gives you a complete Win32 environment with network support, a graphical user interface (800x600) and FAT/NTFS/CDFS filesystem support. Very handy for burn-in testing systems with no OS, rescuing files to a network share, virus scan and so on.
JV16 Registry Cleaner A registry cleaner. Use with caution.
Easy Cleaner A registry cleaner. Use with caution.
AM-Deadlink Removes dead and duplicate bookmarks in most browsers.
SiSoft Sandra System Diagnostics (note only the lite version is free).
SysInternal Lots of freeware programs, too many to list here, under categories: File & Disk; Security; Networking; Processes & Threads; System Information; Miscellaneous.
As of July 18th 2006 Sysinternals has been bought out by Microsoft. This means it's likely that in the near future their freeware may no longer be freeware, so grab 'em while you can.
Graphic Editors
Printscreen A screen capture facility.
Screen Hunter A screen capture facility (note only the lite version is free).
Irfanview A graphics editor package.
The Gimp A graphics editor package. Similar to Photoshop.
Anti Spam
Mailwasher Stops Spam. Bounces unwanted e-mails straight back before it's even left the sender's server. Free version works with only one e-mail account.
Office utilities
Open Office An office suite similar to and compatible with MS Office.
Video utilities
DVD Shrink Enables you to back up your commercially purchased DVD's.
DVD Shrink Tutorials
Audio utilities
Audacity An audio editing facility.
CDex Convert between audio formats, WAV, mp3 etc.
Exact Audio Copy As it says. For most exact copy of audio. Copying can be a lengthy process, but accurate.
Winamp A media player with the emphasis on audio playback. Thousands of skins available.
I've not used the rootkit tools so I am trying both now 
if you are serious and need an OS but don't need to play games. 
