Here's a really simple project that turns an aquarium pump on and off at a specific rate. This is supposed to simulate ocean currents which periodically vary instead of blowing constantly like a mechanical pump does.
The project is built within an electrical box and uses a 558 quad-timer chip as the timing device. I used a monster capacitor and resistor to get time values of 1 to 6 minutes (variable with a pot). The on and off times are independently variable. I used another part of the 558 chip to flash an obnoxiously bright LED. There is also an override switch. The output of the 558 chip drives a transistor, which triggers a solid-state relay. The relay controls power to the outlets.
I added some plans here: http://benkrasnow.blogspot.com/2009/03/diy-wave-maker-plans.html