Radio controlled cars make great robotics platforms.
- They're cheap. $30 will get you a brand new one, and used ones are usually available by the gallon from your local GoodWill.
- They're easy. Generally, they're just two motors and an on/off switch.
- They have a lot of internal space. They're designed to look like a car, so there's usually a lot of empty space inside for shoving in electronics.
- Their battery packs have the right voltages. They're typically AA battery holders, with at least 4 and sometimes more slots, and taps along the way so you can get pretty much any voltage you want (in 1.5V increments).
For my project, I got a little generic RC car from Fry's. It looks good. It has a lot of internal space, and even has a couple of white-LED headlights. I built a little board with an Real Bare Bones Board, and an L293DNE Half-Bridge driver.