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.
Relatively simple to use. Amazing capability to run on Desktop, HTML5, Android, and iOS. My only issue with the engine is the Scene2DUI widgets. They're very confusing and hard to work with.