This engine is very interesting. I had seen it sometime back but seeing it on Steam is what made me really look at it. You can run the editor on Windows, Mac or Linux. The whole thing is BSD Licensed (or MIT) editor, engine and all. It is C++ code and features their own custom scripting language that is somewhat Python-like and influenced no doubt by other scripting languages. Also did I mention it publishes to Android, iOS, Mac, Windows, Linux and even HTML5, and you can also grab the binaries off their side (20 MB ~) or source from GitHub. Neat project. Still havent had the joy of making anything great with it yet. Oh and it does either 2D or 3D.