I used a PID (proportional–integral–derivative) controller to regulate the temperature of my espresso maker. I wrote about it in my book, Made by Hand: My Adventures in the World of Do-It-Yourself. (You can read an except from the chapter on Gizmodo.)
PIDs are often used in Sous Vide cookers, too. (Here's how to make one). If you are looking for a reasonably priced PID controller, here's a new source: Brett Beauregard's open source PID controller, for $85.
Brett's pal, 3ric Johanson (author of Beam Weapon for Bad Bugs in Make Vol 23), says:
My friend Brett Beauregard has been working on this sweet open source PID controller.. and he's finally published information online about it here.
A PID (proportional–integral–derivative) controller is a device which use hardware feedback with an algorithm; this allows the operator to maintain a target value (temp). Cruise control in car is the classic example: Set it at 60mph, and it will increase the accelerator until you hit 60. There are all sorts of things which can go wrong with closed-loop control systems – – overshoot, ringing, bias, etc. As this device is all open source, as it will make debugging these devices much easier.
It's currently on "presale" for $85. I think this is a sweet deal. If you want to make your own Sous-vide cooker, this is the ticket. Go buy one, as I want to make sure this type of hardware is available to the masses. I'm excited to get mine.