I have heard the Crystal Space engine is very good, but I just wanted to throw in that there is also the Source engine (the engine created by Valve Software for Half Life 2 and friends).

Valve has a great Developer Wiki set up, you can check it out at: https://developer.valvesoftware.com/wiki/SDK_Docs

Just another engine to consider Like the Unreal Engine, you must own Half Life 2, Counter Strike: Source, or another game that uses the engine.