I used Box2D in my first android app called Randroid (a simple 2D dice simulator) several years ago. But, virtually, I did not know much about it. After reading the Concerned Joe Devblog, I decide to spend much more time learning Box2D deeply.
I would like to play with Box2D in processing, because I don't need to do much preparation work, and it's easy to tweak with processing. Fortunately, there is a processing port of Box2D called PBox2D at github made by shiffman.
And there is an awesome book named The Natrue of Code that does a great job in teaching you how to make interactive art with processing. It's licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. So you can read it online for free! The Chapter 5 - Pyhsics libraries is what I really need to learn.
and some utilties: