As I have been shopping for a robotics platform to start doing some real robotics work, I think I am focusing on the Roomba robotic vacuum. I was surprised to see that iRobot made it hackable with an interface. While most of the onboard sensors are dedicated to detecting clifts, just as a motorized base with battery, wheel drivers, and encoders it is a bargin at $100. (there are also a number of internal sensors, including voltage, current, and battery temp)
My one concern is that the interface doesn't seem to allow you access to individual wheel control. ou can only give it a speed and a curve value. you can also poll it to see how far it has traveled since your last query.
To get access to the internals, you go through the "Serial Command Interface" SCI detailed here.
A forum for Roomba hacking
Joe Dunfee