The R.U.B.E Box2D editor exports plain-text JSON which can be loaded into other physics engines that have similar representations of bodies, shapes, etc. Here is a partial implementation of a loader for the Chipmunk physics engine.
You can download and run this yourself (Windows, Mac, Linux)
This was an experiment to see how much extra work would be needed to take existing scenes from Box2D and use them in Chipmunk. The shapes themselves seem fine but almost everything to do with joints needs to be re-tuned. The lack of a 'collideConnected' type check to prevent fixtures of joined bodies from colliding, coupled with only a single category bit-flag for collision filtering mean that a lot of careful thinking was required to set up the right flags for structures with a lot of overlapping fixtures.
Another issue was how to add more than one shape to a body without screwing up the moment settings.
I mostly ignored these problems because this was not really intended to be a proper loader, but with a bit more effort I'm sure they could be dealt with. This source code would be useful as a starting point if anyone seriously wants to develop a loader for Chipmunk, so it is included in the RUBE download.