You can find numerous interesting open source projects for Android on Google Project Hosting, but the one which really impressed us was LibGDX.

LibGDX is a cross-platform game development library written in Java with some JNI code for performance hungry sections. (Java Network Interface (JNI) enables one to write native methods to handle situations when an application cannot be written entirely in the Java programming language).

The Graphics are based on OpenGL ES 1.x/2 and supports plethora of classes for 2D and 3D rendering. The developer claims that the graphics have been highly optimized for Android devices {we agree! }. Moreover, it consists of a comprehensive math package for 2D and 3D linear algebra and computation geometry tasks in addition with a JNI wrapper for box2d, which is an open source physics engine.

The documentation is good enough for beginners and some of the games made with LibGDX seem to be really impressive. Android developers should really try this one out!

svn checkout http://libgdx.googlecode.com/svn/trunk/ libgdx-read-only