Martin_H from the Parallax forum has built a cool little robot called Drawbot, it uses a Parallax Propeller Microcontroller programmed using PropForth. The main drive for this robot are the cheap $5 stepper motors that are available on eBay out of China. Looks like it is just powerful enough for this project. There is some loss of position after it draws a lot due to things like wheel slip. This might call for a hacked optical mouse to be added to the bottom of the robot for enhanced position sensing. There are some enhancements to the robot such as remote wireless programming that drives up the price a bit but the convenience of just being able to zap a new firmware file at it without having to plug in is worth it in my eyes. It is programmed in Forth which looks like there would be a bit of a learning curve to it. You can see the code examples on the project page here.