Box2D Physics Engine, and an Introductory Tutorial
Box2D Forum and Manual
“How to create 2D Physics Games with Box2D Library” by Juan Felipe Belon Perez
Erin Catto established himself as a stone-cold boss a few years ago at GDC (sorry, it’s one of my favorite stories), but to get there he had to make Box2D, which has since become perhaps one of the most commonly used 2D physics engine in games.
Mr. Perez’s fantastic tutorial breaks down many of the ways to use Box2D, sorted by what type of effect you’re trying to achieve and containing tons of images, code snippets, examples and links to even more of all of the above.
(Tutorial via Gamasutra)