Our Mac-obsessed brothers at TUAW pointed out that Frets on Fire -- the free, open-source music game that's more than a little bit like Guitar Hero (let's say it was heavily influenced by) -- is now available for Mac OS X. Before you all scream, "Who plays games on Mac OS anyways? LAWL!" we'd like to point out that the game was previously released for Mac OS ... it just didn't work. Now it's been updated and should be working. We've gotten some random crashes on a MacBook Pro, but otherwise it works great.
[Note: Interested gamers should use the Sourceforge mirror; we had some trouble with the download linked in their post.]