shameless plug: here's the paint program I've been working on:
http://evilpixie.scumways.com/
(the most up-to-date code is the 'rgba' branch in github: https://github.com/bcampbell/evilpixie/tree/rgba )
I've had it running on OSX, but haven't tried it recently. I'd expect there to be a few hiccups, but nothing fundamental.