Chipmunk – многообещающий физический движок.
Его основные фичи (надеюсь перевод не нужен) :

• Designed for 2D video games..
• Circle, convex polygon, and line segment collision primitives.
• Multiple collision primitives can be attached to the same rigid body.
• Fast collision detection by using a spatial hash for the broad phase.
• Extremely fast impulse solving by utilizing Erin Catto’s contact persistence algorithm.
• Support for collision callbacks based on object types.
• Impulses applied to contact points can be retrieved after the impulse solver has finished.
• Several kinds of joints available.
• C99 implementation, no external dependencies.
• Ruby extension available.
• Simple, read the documentation.
• Unrestrictive MIT license.

Рекомендую посмотреть демки и примеры. Впечатляет.