Torque 3D, open source, free and a heck of a great community with all kind of tricks and techniques. I also love TorqueScript's C like syntax. The Graphic is awesome as well. Also new things and features through add ons and new releases of new versions are popping up all the time(free as well as commercial). I also find the source code access very comfortable as I can change/add stuff(simple things such as physics and player data) from time to time.
Also the engine just saw the version 3.5 release.
What can I say, I am madly in love with Torque 3D MIT technology.