The Godot Engine has been in development for several years and has a comparable feature set to Unity. The earlier Phoronix article referenced above shares all of the details about the engine that we exclusively broke the news on at the start of 2014. The lead Godot Engine developer has also been engaging with Phoronix readers via our forums.
Juan Linietsky wrote into Phoronix on Sunday night to share that the engine code is now publicly available. The open-source Godot Engine is full-featured and open-source under the MIT license and was pushed to GitHub. Those wanting more information on this open-source game engine can visit GodotEngine.org.