My friend Jesse got me hooked on the idea of cooking sous vide (briefly: using a temperature-controlled water bath to cook food precisely the right amount), and after helping him put one together, I decided to make my own setup.
Rather than duplicating good information that's already out there on the web, I'll just say I (roughly) followed instructions here. I acquired a PID controller, temperature sensor, heating element, and dug around the project closet to find a solid state relay and a switch. Some soldering and assembly, and voila!
I will say that I tried using an aquarium power head as a circulator, with unimpressive results — beyond 50 °C, it stops working. The air pump worked much better (since the motor isn't submerged) and kept the temperature of the water bath within a degree of the target.