LibGDX

libGDX is a professional, high-quality engine for building sophisticated 3D and 2D games, for those who do not have professional budgets. It may not have all the bells and whistles that engines like Unity 3D and Unreal Engine have, but it gives developers the ability to create games that are on par with those created with an expensive engine. libGDX is completely free to use, with no royalties whatsoever. Like with expensive engines, you work off a single source, and build and test your game directly on the desktop. If you are building Android games, you can completely bypass the need for an emulator. If you are building games for iOS, though, you must use RoboVM. While libGDX gives developers a complete set of functionality — which includes audio components, input handling, math and physics libraries, file I/O support, advanced graphics capabilities, 2D and 3D particles, and lots of utilities — it is not as easy to use as some professional game engines. For one thing, it lacks a native level editor, and it also suffers from poor documentation, which is offset a bit by its excellent community resources. Still, if you are a game developer with big dreams and little cash, you should give libGDX serious consideration.