This is my first Box2D game, for Android devices (and my first Android app for that matter). http://www.appbrain.com/app/lofty-ambitions/com.droid13.loftyambitions
I was originally working on another project for Android that uses Box2D and started playing around with the Testbed for ideas. I ended up messing around with the Testbed for quite a while because I thought it was fun and from that, I dediced I would put my existing project on hold and just build a simple game using some of the things in the TestBed I liked playing around with. Although the game itself is fairly basic, it gave me the opportunity to learn more about Box2D quickly and finish something up in weeks instead of months of my free time.
The game uses varying friction, restitution, and mass. Some levels have collision filtering and revolute joints. Working on other levels that will use pulley, distance, and weld joints as well, and anything else I can think of that I can make a points scoring based experience.
A little bit later I will return to working on my original project, a side scrolling shooter with Box2D physics. The experience with this game should help quite a bit...