A few affordable robot board are readily available, I have tried two options:
1. Gertboard with Dagu 4 channel motor expansion
2. DFRobot Romeo V2, which has 2 channel motors.
I find that DFRobot Romeo V2 is much easier to configure. You just basically need to plug it in into your Raspbery Pi and run Arduino IDE to program it, whereas the Gerboard needs several customisation on your Raspberry, and several jumper connections to program its ATMega board to make it Arduino compatible.
The Romeo comes with several Analog ports and Digital ports, as well as 2 motors, and takes power from Raspberry Pi's USB port.
A lot of superb articles written about the two: http://www.linuxcircle.com