The Banshee game engine has just released to 0.4. It’s open source and looks great (imo). I’ve been keeping a close watch on Godot to see what might compete (at least a little) with Unity. This looks like it might be able to as well.
I would take the “open source” moniker with a grain of salt until the engine is out of beta (remember the fact that Xenko and Lightworks went commercial with a ‘free’ version the moment they hit that point).
So it’s open-source for now, but will it stay that way?