The game development website called DevMaster.net has a listing of game engines, some of which are free, open source, or educational in nature.

This is Wikipedia's explanation of game engines in case you are wondering what they are. "The core functionality typically provided by a game engine includes a rendering engine (“renderer”) for 2D or 3D graphics, a physics engine or collision detection (and collision response), sound, scripting, animation, artificial intelligence, networking, streaming, memory management, threading, and a scene graph. The process of game development is frequently economized by in large part reusing the same game engine to create different games."