I am curious how everyone controls the temperature of their primary, especially those that live in the warmer climates. In Florida, where the temperatures can soar and remain in the 90's and above for months, it can be a challenge to keep the house at what is considered "acceptable" temperature for fermentation.
The wet t-shirt and swamp cooler method is probably insufficient for temperatures in the mid 90's.
Controlling fermentation temperature is one of the best things you can do to make good beer!
Like Florida, the temperatures in East Texas get stupid-hot eight months out of the year. Last year I built myself a duck-in cooler powered by a small window air conditioner. I can get the temperatures down to the mid-sixties, and probably further if I take apart the AC's thermostat. See my blog for details.
A fellow brewer recently donated a chest freezer to me and I had a Johnson Controls external thermostat. This combination is ideal for fermenting because the freezer fits a few carboys and the thermostat keeps the temperature within 3° F of your target. I made my first (real) lager, a CAP, in the freezer.
There is also a Low Tech Lagering DVD. I have not seen it yet, but I trust that James over at Basic Brewing Radio knows what he's talking about.
One of the cheapest solutions I have heard of is to immerse the carboy in a Rubbermaid water cooler filled with water. The water will act as a great insulator. To heat or cool to keep the temperature within the range you are looking for you can use a fish tank heater and ice packs. This method obviously means you will be needing to check on your beer quite often, but more so if you are trying to cool it with the ice packs (every 4 -6 hours). The heating part of it requires little attention, since you would get to set the thermostat. The thing that is most appealing about this solution to me is that it does not take up much more space than the carboy normally did. I have never actually done this, but the method was recommended to me by my local homebrew shop.
Rubbermaid water cooler $68: http:// www.amazon.com/Rubbermaid-1610-01-11-DISC-10GAL-Orange-Cooler/dp/B00002N9F4
Aquarium heater $22: http:// www.aquariumguys.com/ebojager4.html
Here is a link to a more automated version of this solution: http://www.homebrewtalk.com/f51/water-fermentation-chiller-81501/
Build a fermentaion chamber.
One of the best homebrew purchases I ever made was a dorm-style mini fridge (without freezer) that I removed the shelves from to create the perfect-sized fermentation fridge. This is controlled by an external Johnson controller, allowing me to dial in whatever fermentation temperature I want. It's ideally sized for buckets and carboys.
The fridge cost about $200 new, plus you'll need some kind of flat material (I used plexiglass) to secure the rubber seal back onto the inside of the door after you remove the door shelves.
Now the only problem is I wish I had a second one for double batches or for lagering. I bet you could lager at least 2 or maybe 3 batches in cornies at one time. Edit: I can fit 2 cornies in my Sanyo 4912.
I'm in Vegas and have the same challenge. During the summer my house is usually at or near 80 degrees inside. I intend to eventually move to a chest freezer and thermostat, but for now I use a low-dollar method similar to what Jordan recommended. I just put the carboy in a Rubbermaid storage bin (instead of a cooler), fill it half-way with water, then drape towels/rags/shirts over the exposed part of the carboy making sure both ends hang in the water. If normal evaporation isn't doing enough to keep the temp down I have a small fan that boosts the effect. Two problems: you have to make sure the water level doesn't drop too far and this may not work well in FL due to the high humdity.