The libGDX project is a cross-platform game development library written in Java, with some JNI code for performance hungry sections. It abstracts away the differences between writting desktop, Android and HTML5 games based on standards like OpenGL ES/WebGL. Applications can be prototyped and developed entirely on the desktop, then only 6 lines of code are needed to run your app on Android or HTML5.
After using this engine for well over a year for multiple games and test demos, I can honestly say it's grown to an amazing state. It provides a natural feel for object oriented programming and game design, making it a high-level framework that one can jump right into using, unlike low-level alternatives like SDL. Additionally, the freedom it provides programmers with still providing access to low-level functions and opengl wrappings make it ideal for anyone familiar with Java who wants to fiddle with all degrees of development.
It addresses short-comings of the language's api, it's highly portable and cross-platform, and best of all it has an extremely active development team. A few years ago it was just a little LWJGL wrapper, but now it's truly a project worth recognizing and is a dream to work with. XNA/Monogame barely even compare to the amount of out-of-the-box features LibGDX provides, as well with the amount of control you get from it.