Chipmunk2D

Scott Lembcke wrote Chipmunk in the second half of the 2000s, starting from Erin Catto's Box2D Lite but choosing C over C++ and a leaner API. It handles rigid bodies, joints and collision detection, using spatial hashing or a tree for the broad phase.

During the iPhone gold rush it shipped with the Cocos2d family as one of two built-in physics options, next to Box2D. The C interface makes it trivial to bind from other languages, which is why it keeps turning up in places you wouldn't expect.