I've got a shopping list for doing this since I got my Keezer. Going to be using an Arduino microcontroller and a couple of expansion boards to start with to control the compressor (one custom board for a relay switch, and another for a few temperature sensors).
My first iteration will include an LCD temperature display, and a feature for graphing the temperature over time for brews (with a sensor inside the carboys themselves).
Second iteration will include pressure sensors so I can have a display of the levels of the kegs inside the keezer after a little calibration.
www.sparkfun.com = win.