I just want to point you to a library I've been developing during the last few months, mostly to use for my projects, but I consider that is useful enough for general use.  It's currently in it's fourth release.

Features:

• Based on Verlet integration.
• Fast Relaxation solver for points. Offers two modes with tradeoff between speed and accuracy.
• A solver for groups of 'charged' particles, i.e. particles that are attracted/repelled from each other based on their 'charge' property.
• A solver for twin groups of charged particles, i.e. each particle of one group will affect each one of the other, but none of the same.
• A simple constant force modifier.
• Unyielding points with drag multiplier for each axis (X,Y,Z), in order to constrain to different degrees depending on the axis (i.e. points constrained only on X axis etc.).
• Extensible base solver class that supports multi-threading. New solver or modifier classes can be developed on top of that without the need to mess with threads etc. (will soon post a tutorial on how to do that).

• Proper weight support for individual points.
• More solvers like projection to surface, collision with planes etc.

While this library has been around for some time I am pretty confident that bugs can be found and documentation is far from perfect. However I am actively engaged in developing it further. And of course it's open source, released under the New BSD License.

Check it out on my website or on Google Code   UPDATE:  fvlib now lives in Github! the Google Code repository is obsolete and will be phased out soon!

Thanks