- Cygwin: Makes any Windows machine able to run a linux-like terminal
- Caffeine. Addicted.
- Ethereal: If you are a geek, you need this. Packet-sniffing goodness
- Trillian. Yeah, GAIM is open-source, but Trillian is just that much slicker
- Firefox. Runs anywhere; need to know more?
I know that most of these are more for the geek side of the house, but a lot of my traffic comes from those folks.
Leave your additions in the comments.