Maxima -- Blender 3D Game Engine

Maxima

Blender Artist thread:

The Blender 3D game engine can be used to make and play computer games.  Let your friends download and play your computer game.  Use the Blender 3D game engine to model game characters and 3D environments to create casual video games, first and third person shooters, role playing computer games, racing simulations video games for download.  Sell your games or let people download them for free.