A fermentation freezer or fermentation refrigerator is a freezer or refrigerator that has been converted to a temperature controlled fermentation chamber. Controlling the fermentation temperature of your beer is one of the best ways of improving the quality and taste of your beer. A fermenting beer generates a great deal of heat, and if you live in an area where room temperature reaches 75° F or more, you can have a very difficult time keeping your fermenting beer at an optimal temperature.
If a fermenting beer reaches too high of a temperature you can get some nasty off flavors and potentially kill your yeast. I live in Southern California and when I first starting out in home brewing, I had to try all the tricks to keep my beer cool; I placed my carboys in a plastic tub filled with water, I covered the carboys in wet towels and had a fan blowing on them, but no matter how much effort I put forth, I could not regulate my fermentation temperature nearly as well as my fermentation freezer does.
Setting up a fermentation freezer or refrigerator is easy and can be inexpensive as well. I purchased mine when Lowes was having a 20% off appliance sale for around $400, but other people I know have gotten amazing deals on Craigslist for as low as $75. All you need besides a freezer or refrigerator is a temperature control unit like a Johnson Controls Digital Thermostat which you can find here.
The freezer that I use is a Frigidaire 14.8 cu ft LFFN15M5HWF, which works great for my needs. It fits two large 6.5 gallon carboys, 4 pin lock conversion 5 gallon kegs that I use for secondary, and about twenty bottles of beer (a.k.a. my emergency stash). I use a Johnson Controls Digital thermostat to maintain the temperature. Best of all, since I use pin lock conversion 5 gallon kegs, which are shorter then ball lock kegs, I did not need to build a collar onto the freezer for additional height. The only complaint that I have is that on occasion I get more humidity then I would like. To combat that, I periodically wipe the inside of the freezer down with a dry towel to remove any moisture and also use a product called DampRid, which helps absorb the remaining moisture.
Image of the chest freezer that I use for my fermentation chamber: