I am looking for a free game engine for a beginner (that's me) on a Mac. I have programming experience but I would rather have a graphical interface. Also I would need a large number of models and textures to get started. I need to use it for six months (the duration of a class I am taking) so a 30 day trial would not be a viable solution.

Any suggestions?

http://www.idevgames.com/forum/showthread.php?t=17953

For your purposes I would recommend Dim3 as a start, and looking around the 'net for asset packs will yeild something. A certain degree of self-tuning will be needed though.

Try Platinum Arts Sandbox.

If you are interested, I can see about sending a copy of iGame3D over iChat, for some reason my FTP keeps canceling the 635 Meg zip at about 90% complete.
It's not nearly as fancy and polished as Sandbox but there is a ton of content and I could use the motivation to do something with it again.

Email me at bill at igame3d dot com.

What kind of "engine" are you looking for? It can mean a lot of different things IMHO.