The Dualshock 2 controller is a great addition to your next Arduino powered robot. However, it can be tricky to convert the 3.3 voltages of the controller, to the Arduino’s 5-volt system. Check out the link for a great solution to this problem, including the schematics and code.
A Playstation controller has two analog mini sticks (Dualshock and DualShock2 only), 12 pressure sensitive buttons (Dualshock 2 only) and two digital buttons. All these inputs can be made available to the Arduino via only 4 pins (three of which can be shared with other functions, if planned carefully).
Read more about How to interface Arduino with a Playstation controller [Arduino Forums]