PieterBot (yes there are many out there :)

thingiverse

A small robot prototype featuring continuous-rotation RS2 servos (e.g., from Conrad), a Tower Pro Mini Servo, a 3 AA battery holder, an Arduino Nano mounted on a small breadboard, and an ultrasonic sensor from eBay or DealExtreme. Modify the RS2 servos for continuous rotation and print various parts including base, battery, and servo holders, as well as casters and a potential head from Thingiverse. Assemble the components with M3 bolts, glue, and superglue. Wire everything up, upload the provided basic program (PieterBot.zip) to the Arduino Nano, and improve upon it for better functionality; consider adding an extra ball caster, a better battery stop, servo holder, software, or even a laser for no specific reason. A video of this prototype in action can be found here: http://www.youtube.com/watch?v=5CfzsNUcKHk, and another without the head here: http://www.youtube.com/watch?v=4UULxM7JtcY. Remember that this project is still a work in progress as the creator focuses on building a custom quadcopter.