I see that "obijuan" has designed
REPY-1 modules on Thingiverse that can be printed with a RepRap.
Like most
modular robots, it appears utterly useless when you have one module by itself,
but sufficiently large numbers connected together can do some pretty cool things.
Since each module does so very little -- controls a relatively slow mechanical motor, does a bit of sensing, does some communication, and forwards messages in both direction -- it seems likely that a relatively small, cheap, single-core microcontroller in each module may be adequate.