Check out my Arduino Motor Shield, it's on a 4WD Robot car controlled by an Arduino Uno. This is part of an ongoing robot project, using bumper sensors (microswitches), scanning ultra-sonic sensors (SRF05), WiFi camera, bluetooth, RC and lots more. Note it is advisable to cool the shield's main chip (eg heatsink).
Code is Here:Gampa Geek's Google Drive
Showcased Here: http://www.rapidonline.com/arduino/your-projects
Lots's more robot code to come as I crack on with this project.