Along with dan, I'm posting some of my more used apps under OSX:

Eudora is a pretty good alternative to Entourage but it's still in beta.

The E language works under OSX now.

Castle Wolfenstein multiplayer beta2. This is the bone thrown at us while Windows users get to play the full version.

Snak: a pretty damn good irc client.

HP48: HP released the ROM to this modern class to the public, and an emulator is available under the GPL

Omniweb: a good alternative to Internet Exploder

Quake 3 Both Arena and Team Arena have been played a lot lately.

MacMAME has been giving me lots of trouble but I'm sure things will be fixed soon enough