The ping pong ball launcher was originally designed to be an arcade game of sorts, in which you'd have to hit the balls back into a chute with a lever-paddle-thing. I was to build it in a week, which could have worked had I had a definite plan, but as it was, I was designing it as I was building it, so I never quite finished.
The Brains of it all is a funny little microcontroller called the Super Cricket. This is driving lever on a geared motor which pulls back a spring-loaded flapper. As the lever rotates, it breaks the beam of light between an LED and a photoresistor, which the Cricket detects and in turn triggers a servo to rotate a wheel with a whole cut in it to transfer a ping pong ball from the storage tube to the launching area. This is all timed so that when the ping pong ball is in position, the flapper is released, launching the ball.
Congrats if you followed that. If you didn't, here's a video to better show what's goin' on.