TinyDuino and TinyLily The former just smaller than a quarter, the latter just smaller than a dime, yet they have pluggable modules ("shields") for a variety of features; the "Robotics Kit" pledge level is a couple of TinyLilys with a bunch of motor-drivers (1.8A H-bridge chips on washable circuit boards (pledged, funded, still available)
They'd pair really well with RadioBlocks which are a bit bigger (no handy coins for scale) and do 802.15.4 mesh networking using Open Source firmware, and draws 14mA at full power - so you can run it off of the data pins of another device, as long as one of them is set to "1". (pledged, funded, less than an hour to go!)
It's nice to see the modern "legos of computing" actually getting down to the size of lego bricks, and getting cheap too. Not quite at the level of "stick some coin cells, chips, and humidity+sunlight sensors into ping-pong balls and shake a bag of them over your garden" but definitely heading in the right direction, where it's not completely ludicrous to think about "ubiquitous computing" ideas like that at the hobbyist level...