Easiest way is probably the Arduino Duemilanove, about $30.00 these days. It's a good starter MCU, and comes with an ATMega328, which is equivalent (quadruple the memory) to the controller used in the $50.00 robot tutorial, and has a FT232 UART to USB converter built in. Of course, not knowing what sensors you have, or the motors, I can't really recommend much there. Any other microcontroller you have experience with will work too.