Here are my fav's:
Proj. Gutenberg - Over 12,000 books online free! Authors ranging from Da Vinci, Einstein, Dickens, etc!
Firefox - Firefox!!! Get the BugMeNot plugin, also
UltraVNC - access your PC from anywhere!! Even a java-enabled browser!
Packetyzer - Awesome frontend for Ethereal
Gnump3d - You're on MP3/OGG server
AquaXP - make your Windows look like Mac. Cool as hell!
Black Viper - Configure your Windows 2K/XP services. I recommend this to ANYONE who uses 2k/XP. Disable all those useless services!
Xbox Scene - xbox stuff
AIM Mutation - Cool tweap for AIM. Get the Smiley Plus plugin and get over 100+ new emoticons!
Unattented XP CD - install XP w/out being at the comp
How is chocolate made?
CpuID- get cpu-z. tells you everything about your CPU, mainboard, memory. Examples: tells you how many slots you have and the kind (ex: 2 DIMM slots)
TuneXP - tweak all aspects of your Windows system. Increased my system speed! Just follow the documentation
Belarc Advisor - complete audit of your PC. Also try Everest Home Edition or PC Wizard 2004.