The Godot Engine:
http://www.godotengine.org/wp/
It is Free and Open Source under the MIT license and is very feature rich. It has only recently reached their first stable version. However, This shows as much potential as Unity3D did a long while ago.
Did I mention it is Free and Open Source?
Great find! It looks like it's coming along with some promise. I'll keep an eye on this, can't wait to see how it develops.
This gives a whole new meaning to "waiting for Godot".