If you've not already heard of it, PlaneShift is a free cross-platform fantasy 3D MMORPG in development. It is produced by a team of volunteers aiming to create a fully free virtual fantasy world in which a player can start as a peasant in search of fame and become a hero. The game uses the Crystal Space 3D SDK, the source code of the engine is Open Source under GPL and servers and bandwidth are donated by sponsors... More on
http://www.planeshift.it.
The team currently needs support on MAC OS X and is looking for people with building/programming knowledge on MacOS willing to help porting/testing the game on Mac. If you are interested or would like to have more information, you can contact the PlaneShift team on irc, server freenode, channel #planeshift-build or pm me.