The Unreal engine is very popular, and freely available (obviously license fees do apply when its being used commercially).
It also links in very nicely with Maya via a plugin.
Look here https://udn.epicgames.com/ for more info/tutorials/downloads
Other engines that are widely used are the Quake 3 (getting a little long in the tooth now) and the lithtech engine which is comparitivly cheaper to the others (about $250,000)
hope this helps

btw: your not from Immersive Education are you? just curious