One of the projects I've worked on in my spare time is a game- and physics engine for android. It's written in Java using OpenGL ES 2.0. I was originally planning to make a game, but I had to much fun writing just the physics engine that development of the game never really got of the ground.

The engine is meant for 2D games. It can render sprites in OpenGL using quads and really simple shaders. It allows for transparancy by correctly ordering the objects in the scene. The physics engine is polygon based (not pixel based), and can currently detect collisions between convex polygons, circles, and combinations of these objects with any rotation and scale. The collision resolution (what happens when objects hit each other) is still slightly unstable sometimes for fast objects, but works most of the time. I plan to finish this project soon and put the source on github.