There are a lot of neat projects that I use or follow. Here is the list of the ones I can remember right now.
Boxee
m0n0wall
Untangle
DD-WRT
GNS3
Pidgin
Notepad++
Zenoss
Smokeping
Additions to the list:
Cacti
7Zip
VLC
WinSCP
Eraser
TightVNC
(omitting the projects that everyone already knows about e.g. Ubuntu, Webmin, etc.)
I love finding a cool piece of software (or hardware) to play with.
Here's an extensive list from Lifehacker:
http://lifehacker.biz/articles/best-open-source-software/
