I made a controller for my chest freezer using a microcontroller, an LM335, and a relay.
I've been thinking that I will make a smaller / lower cost temp controlled water bath using a Peltier cooler in an 18 gallon tub. Rough back-of-the-envelope calculations* imply I can easily keep it down to lager temperatures with a couple 300W Peltier junctions with ambient temps around 80F. If/when I test this out I'll post a thread about it.
*Assume we have 20 kg (about 5.5gal) of a 1.070 beer that ferments out completely in 48 hours (this overestimates the power output), and that it's fermenting at a more or less constant rate (not quite true, but close). This means that there are 1300 g of sugar being processed. 1 g of sugar metabolized completely results in ~17 kJ energy released, so in total 22 MJ will be released during fermentation. If this is spread over 48 hours, we have a constant power output from the fermenting beer of about 130W. In addition, there is some thermal loss to ambient. With reasonable approximations for exposed surface area, with a 20*C differential we'll have to overcome about 300W of heating from the outside world.