libGDX[1][2] (it's not an engine) is a great cross-platform Java library that allows you target the desktop (Win/Mac/Linux/etc), Android, HTML5 (through GWT), and now iOS all with the same codebase, built on standards like OpenGL ES/WebGL.
Mario Zechner (the main guy behind libGDX) wrote an excellent book[3] last year on Android gamedev that has you start from the basics up to creating your own game engine and games.