Another impulse based 2D physics engine written in pure .NET. Has an excellent API and can be integrated in a matter of minutes.

A 2D physics engine completely written in C#

Portable .NET bindings for most of the libraries popular in game development, including DevIL, OpenGL, SDL, Cg, ODE and OpenAL. Saves you from the work of writing custom wrappers and updating them on each and every .NET platform, though you will still have to cope with the procedural API these libraries provide.

Open source dynamics simulation library. This is the most widely used library for realistic physics simulation in games that can't afford the expensive Havok engine. ODE has a large developer community and is, if correctly used, able to deliver believable and fast physics.

Closed source dynamics simulation library for games. Less known, but in many cases easier to implement and get right than the complex ODE engine.

A closed source dynamics simulation library for games. This library is said to provide better results than many of the other libraries out there and is very easy to integrate, too. On the other hand, it's not available for free.

Ageia's PhysX SDK (previously known as Novodex) is a direct competitor to the widely used Havok physics engine in commercial AAA-titles. Ageia's SDK is available for free and besides to full range of rigid body physics, it also has some neat features like fluid simulation. Using this SDK also enables you to harness Ageia's PhysX hardware physics accelerator.

Bullet is a project by an ex havok employee. It has developed into a real physics engine that offers continuous collision detection (in contrast to most of the other physics engines which let objects penetrate each other and then try to compensate by moving the objects around) and a solid set of rigid body physics functions.

An upcomer between the currently established physics solutions. This library claims to do swept collision detection, meaning that even the fastest moving objects (bullets for example) would not be able to 'tunnel' through other objects in low frame-rate situations.

A thin wrapper around Tao.Ode to provide ODE with more .NET-like interface in .NET languages such as C#.