I built my second one yesterday. I use this to monitor my tank temp and turn on my fans when it gets too warm. The fans will turn off when the tank has cooled sufficiently.
This is the box I used. (About $7 at radio shack)
I used a dremmel to make the cut outs for the outlet and the microcontroller.
This is the controller (purchased on ebay) (~25)
Besides wire and a plug you'll need these (I just used a spare computer power cable):
Use this schematic (I skipped the heater connection and did not break the tabs on the outlet (so i ended up with 2 plugs for cooling). I used 4 (~5 inch) wires for my box and used 1 wire nut. If you wanted to hook up the heating side you would need two more wires.
This is the source of the schematic and more info on other people's builds.
Here's the front (note: it is in Celsius)
Here is the back