i'm surprised no one has mentioned "Blender".
http://www.blender.org/ this is an open source, freebie. it's been around for a while now.
and if you're interested in ID software (games like Wolfenstein, Doom, etc, etc.) there are 3d editors, free, for those. the one that sticks in my mind that i've used on occasion is, gtkradiant. i believe this is the current address of the official release from ID software. oh, it's open source too:
http://www.qeradiant.com/
you'd be surprised how good a 'game' editor can be.