Possibly one of the most powerful and popular game engines has been released to the public free. The UDK (Unreal Developer Kit) allows people to use the awesome Unreal Engine that powered some of the best titles out, including the recently released Batman - Arkham Asylum.
http://www.moddb.com/engines/unreal-engine-3/news/unreal-engine-3-free-to-indies
EDIT: Ninja'd by JeffMOD in the shoutbox =( Dammit