I dunno about osx, but there is a cool program called leafdrums (
http://www.leafdigital.com/software/leafdrums/) for Windows. You do have to pay for it I think, but i don't think its much. It does not come with any drum sounds, but you can get a really cool fully recorded free recorded drumkit from
http://www.michaelkingston.fi/kingstondrums/ called Easy Rider. You just download the rar file with the .WAV files in, unpack, and import to the wavs to leafdrums. I found this worked like a dream.
