PhysicsInvaders is a project mainly written in Java, it's free.
Thought I'd make a remix of Space Invaders using html5 canvas, GWT, and box2d.
You'll need to download gwtbox2d, export it as a jar, and include it in your build path before this will work. Also, don't remove /java/org/jbox2d/emul/java/lang/* from this project or it won't build. You'll need GWT 2.3.