Divx and xvid have QuickTime plug-ins, which means you can use them in lots of OS X apps. Or, as others have said, there is VLC. Though if you drink the Apple kool-aid you won't like open source apps that don't behave like native OS X apps. (Full disclosure: I've been successfully brain washed by Steve Jobs in the past 6 months.)

http://labs.divx.com/archives/000057.html
http://www.versiontracker.com/dyn/moreinfo/macosx/25128

Those are the codecs. Believe it or not, I actually like the new Real Player for OS X. I'm not sure if it uses QT plug-ins or it's own codecs. But it played the things I've tried so far.

Here's a new player that I haven't tried yet that looks promising. It works with QT plug-ins. QT itself is a pretty weak media player. But like Windows Media codecs there are other front ends that have access to the video playback engine.