We built a robot to play air hockey that turned out to be pretty effective. We needed a very highspeed camera to make sure that we were able to track the puck as it was hit around the board, so we used the PS3 Eye camera capable of running at up to 187 fps (Sorry Kinect).

For hardware we used generic aluminum, wood, and hobbyist motors, but for our gear and pulley system we used a synthetic lightweight chain and hardened aluminum to keep the weight of the arm low and still maintain accuracy.

On the control side we used Simulink (Commonly known as the bane of my life) for conversion of X/Y Coordinates to motor positions, ensuring that the paddle moved in a straight line, and feedback to the gameplay logic as to where the paddle is located.

For hardware we had Quanser Q4 board and xPC target whick made interfacing using Simulink wonderfully simple (Though a lot of the ports ended up dead and the amps were kind of falling apart by the end of the project).

Here is a video that goes into detail and shows it in action kicking but:
https://www.youtube.com/watch?v=NI1gYR5XpMk

Thoughts? We came in second place, sadly, to a team whose robot mostly just ignored the puck and defended their goal. Unfortunately they were so good at that, that we couldn't ever get a good shot on them!
I'm pretty proud of the finished product though, but I can't help but think of a hundred more things to try to improve the behavior.