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I'm using a wonderful game engine that can export to Android, iOS, WIndows, MacOSX, Linux, HTML5, 3DS.
You can develop on it if you have Linux, MacOSX, or Windows- although I'm sure if you compile it from source it would work on other *nix, like FreeBSD. (Linux support is the selling point for me...)
It's Free/Open-Source (MIT License), 2D/3D, Unity-like, has a scene/node system, and it's homebrew Python-like programming language.
There may not be a wealth of documentation, but there's just enough.