Last year I bought a robot kit from www.wrighthobbies.com. I bought it specifically to hack but never got around to it.
You can buy the kit here for$10.00 at: http://www.kitsusa.net/phpstore/html/21-890S-R3D3-LINE-TRACKING-ROB...
It's a two wheel line follower about 6 inches tall with a very nice two motor gear box. It's also a very nice looking chassis. The robot is easy to assemble, I built it and soldered the board in less then a hour.
What makes this robot so hackable is that it comes with a schematic for the board, and the board traces are easy to get to.