Farseer a open source physics engine which can be implemented in .NET Framework. Farseer can be implemented in XNA (sadly only for 2D Application), Silverlight/WPF, and brand new mobile patform Windows Phone 7. This engine is quite interesting, it has many features of physics simulation , for examples, collission, rigid body, ragdol, dynamics, friction, etc. You can download this engine here.
Jensen Walker, it is a simulation of Jensen’s spider robot.
Familiar example, a physics car racing.
Advanced dynamics, hey, it’s World of Goo !