Arena and Lucas chess are good. I like Lucas for it's simplicity though. And it is fun for my kid to play with. It comes with a tutor option also.

http://lucaschess.pusku.com/

http://www.playwitharena.com/?Download

Edit: Not sure if all of these are Open source though. But they are free.

Try 'Scid vs PC' or ChessX. You'll get a good-looking GUI, plus some cool database features.

http://scidvspc.sourceforge.net/

http://chessx.sourceforge.net/

A cool feature people may not know about - many of these GUIs allow you to play two engines against each other. It's fun to watch two strong programs slug it out. :)

As far as I was able to tell, when I was interested in chess a few months ago, there is no acceptable Mac GUI for Stockfish.

It also works with GNOME Chess.

Google "Scid vs Mac"