SomethingUnreal has built a simple system to demonstrate how a PID Motor Speed Controller works. The system is spinning a piece of cardboard on a toy DC motor. The motor is powered by an H Bridge. An optical sensor provides feedback to the system so that it can see the actual speed and adjust the H Bridge accordingly. The software running on the PC is very nice as it allows for lots of variable playing and graphical output so you can see what the system is doing.