Just playing around with my new machine (iMac G5 20 inch, 2GHz) and OS. Here's a list of things you will probably want to install if you're in the same situation:
Firefox (with extensions Document Map, AniDisable, Flashblock, textareatools; Enhanced Bookmark Search does not seem stable, unfortunately)
Unix heads will want to get Fink, which I found to be superior to DarwinPorts, but that's just my particular mileage. I also like Audio Recorder.
You'll also want to switch off auto-installing of Dashboard things (security vulnerability! find it in Safari settings), make Firefox your default browser (ironically, this is also done in Safari settings), get rid of the Dashboard icon, (more to follow...)
You may also want to get the following video players:
MPlayer OS X 2
Windows Media Player
I don't know what the deal is with RealPlayer at the moment - I don't seem to need it...
These not-strictly-necessary-but-currently-more-compliant messengers (Adium actually supports both protocols):