Here is mine so far:
* Adobe Reader (most people know this one, I think)
* Zip Genius (allows you to open virtually any type of compressed file)
* Open Office (basically replaces Microsoft Office)
* Avast Anti Virus (self updates at least twice a day, not a resource pig, found viruses and trojans that the big names did not, P2P protection, scans MSN/ICQ, realtime protection, email scanner, different skins)
* Microsoft anti-spyware Beta (yep, believe it or not, it's actually quite good)
* Lavasoft's Adaware
* Search and Destroy Spybot
* Spyware Blaster (doesn't actually scan, but rather, stops most of the malaware crap from installing on your machine in the first place)
* Gmail (best email I've ever used)