So you've probably seen this story about the table that keeps a ball in the center by using a touch-screen to sense where the ball is and tilt the table accordingly.
In the Reddit comments for the story, someone linked to the PID Controller page at Wikipedia, and wow has that thing gone uphill since the last time I looked at it.
The most interesting thing I learned by reading the (new version of) the page came from an off-site link to a PDF file about the control system for a little self-controlling helicopter project. It turns out (page 22) you can build a PID controller from an op-amp, two resistors, and two capacitors. You do have to do some math on the resistances and capacitances to calculate Kp, Ki and Kd. But even so, what a cool little trick!
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OpAmps - is there anything they can't do?