I just discovered this list of open-source applications for the Mac. It's a great list and has all of my favorites (Adium, Handbrake, VLC, SpamSieve, NeoOffice, etc.), as well as a range of new ones that I've yet to try. Like AppFresh, which automatically keeps all your applications updated.
The best platform, with the best applications. All 100% open source.
UPDATE: In looking through the list more closely, several aren't open source. Still, the list is such a good one that you should have a look.