pymunk is a wrapper around the 2d physics library Chipmunk
The reason why I started on pymunk is that at the time (around summer 2007) there wasnt any 2d physics library available at all for python programmers, the best one could do was to use pyODE and then constrain the world to 2 dimensions, not optimal at all.
It has been a slowly growing project, at first it was a just manually-edited auto-generated low-level binding, but from about spring 2008 I think its in a usable state even if its still not perfect in any way.
It is also interesting to see that during the time Ive worked on pymunk a couple of other 2d physics projects have started for python, for example pyBox2d/Elements and the pygame-physics projects run as GSoC project.
For more information about pymunk see its main page at googlecode: pymunk
todo: add a couple of screenshots