There are several options. I currently have the Johnson A419 digital controller on my kegerator, and am going to buy another Auber 2362 PID (same as the ones in my control panel) for my fermentation fridge.
The Johnson A419 is pretty easy to setup, make sure the jumper is in the "cooling" position, and then plug it in. You can set your cut in/cut out temp, and your differential then you're set to go. For mine, I set the "cut out" at 37F with a differential of 9F. Also, the "ASD" setting is the minimum time between cycles, so if you don't want your compressor starting up more than once in a given amount of time, set that time as "ASD." I set mine at 12 minutes, some set it at as low as 5 minutes. This controller is a bit more expensive than using a PID.
Code_Rage wrote a nice "how to" for seting up the Auber PID 2362 for a fridge here. This PID is a little cheaper than the Johnson controller, but requires a little wiring and bit more initial setup.
Sorry if I confused you. Ask any questions you have, and we'll try to answer them.