Summary

This Project was created around tiny planetary gear motors found on eb** (http://www.ebay.de/itm/361082979291?_trksid=p2060778.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT). They cost less than a coffee a piece, so by using the 3D printing technology to print custom wheels and a chassis it should be possible to experiment with little robot swarms on the cheap or simply build a single lil robot to experiment with. The Turnigy Nano Tech 170mAh Lipo cell that this chassis was designed for is also very common and easy to source.
I'm planning to boost up the battery voltage to 5V and to use the L293D dual H-bridge to drive the motors. I used an Atmega 328p w/ Arduino bootloader as the microcontroller, but there's tons of flexibility here. There are plenty of holes you can tap with an M3 tap to attach PCBs, sensors...
The 40mm wheels can accept O-rings with a thickness of about 3mm to have a better grip. The oval cutouts in the wheels can be detected using reflectance sensors, which can make up an encoder to determine the traveled distance.
A common high ball caster (http://www.watterott.com/de/Ersatz-Ball-Caster-mit-127mm-Kunstoffkugell-3pi)
can be superglued to the bottom or you can design your own.