Dynamechs

Dynamechs (http://dynamechs.sourceforge.net/) is an Open Source multi-body dynamic simulation library for multiple operating systems. It is built on top of OpenGL and features the following:

• Multiple joint types supported
• Support for serial chain and tree structured mechanisms
• Support for closed chain systems
• Support for fixed or mobile bases
• Support for lumped parameter hydrodynamic effects such as drag
• Can be compiled for single or double precision computations
• Multiple numerical integrators

Although nowhere near as an impressive feature list as the commercial tools, it's still a good set of features, and a good starting point for a developer. The major drawback to this solution is that it does not feature any collision detection.