Bullet is a physics engine which stimulates collision detection, soft and rigid body dynamics, soft body support, and more.
Box2D is an open source C++ engine for simulating bodies in 2D that has features in collision, physics, system, testbed, and documentation.
Havok is a collision detection and physical stimulation technology for interactive software for digital media creators in games and movie industries.
A modular, extendable, and easy-to-use physics engine for javascript
Matali Physics is an advanced, multi-platform, high-performance 3d physics engine intended for games, virtual reality and physics-based simulations. Matali Physics and add-ons form physics environment which provides complex physical simulation and physics-based modeling of objects both real and imagined.