FFview - an image viewer. Great for large, organized photo collections (see my iPhoto rant earlier). Designed for reading manga.
MPlayer OSX 2 - _the_ movie player. Handles everything, no questions asked. I don't even bother with that quicktime shit.
Adium X - IM client. Connects to all the networks I care about. Small and unintrusive if you change the dock icon. Themeable. Very nice program.
Frozen Bubble - game. More like an addictive, polished game. A must have for any platform.
Temperature Monitor - self explanatory. I leave this running, because I like seeing graphs of sensors. This guy makes lots of other great software too. Be sure to check out TinkerTool.
Ogg Vorbis Quicktime components - lets you play