Flash ActionScript 3.0 port of Erin Catto's 2D Physics library Box2D.
While very quick and powerful; if you are a user of this library, and not a developer of it... you get screwed when it comes to documentation. Class names can be incredibly atrocious, and nearly everyone starts with 'b2' for no apparent reason. (examples: b2AABB, b2AABBBodyIterable b2OBB, b2Vec2, b2Mat33, b2TOIInput) Documentation for a function such as 'SetSprite' is usually nothing more than 'Sets the Sprite.' and you are lucky if you even get that. Occasionally you'll get parameter documentation. I'm not aware of any HTML Documentation, which could be easily generated using ASDoc after the code is properly documented. It just needs a good clean up.
After a new installation of windows one of the first tools i always install+)