I turned my Raspberry Pi into a robot, controlled by a Wii nunchuk. It's surprisingly easy to do - at least, surprisingly to me, who has not previously made a robot. But because it's surprising, it might help others to have a guide, so here one is.
I'm linking to SKPang for most of these, but Sparkfun and Adafruit would be good places to look if you're in the US.
(If you're in the UK, a word of caution - I bought motors and some other stuff from Sparkfun to save £7 over SKPang, but the package got stopped at customs and I had to pay £4 VAT and £8 handling fees. My understanding is that this will only happen on packages whose contents are worth more than £15, but you'd be a fool to trust me on this. It didn't happen when I spent £20 at Adafruit or £5 at Sparkfun. YMMV.)
- Raspberry Pi - for the robot logic. An Arduino or similar could be substituted (though the software would need to be rewritten).