The key to any good beer system is controlling the temp during fermentation.  To that end I have spent the last year or more in designing and implementing this temp control system.  The first version used a small college fridge as the cooling source but it simply could not keep up.  I had an old dehumidifier that I modified to chill a 35% glycol/water solution. 

Put simply the system uses the chilled glycol solution through a chiller coil in the fermenter to cool the wort and the aquarium heater to warm it, depending on what the external conditions are.  it can hold the wort at 34° F indefintely.