Programmer Deryck Williams has created (or is still in the process of creating...) a large, ever-growing robot based on a four-wheel drive chassis. Using more than one Arduino board and several custom protoshields, his robot not only moves about but can monitor battery voltage and current draw, search out for bright light, and has a claw on a pan/tilt base. And of course there are many flashing LEDs. Interestingly the multiple Arduino boards communicate with each other via the I2C bus - thus adding a form of intelligence.
When recreating a project such as Deryck's mega robot, consider building the external circuits on a ProtoShield - it makes your project all the more professional, and easier to enclose. We have a range of shieldsincluding basic PCBs, short versions and our ProtoShield Pro with SMD LEDs: