I don't remember how I stumbled across this, but the idea hails entirely from Stephanie, the self-proclaimed "Crock-Pot Lady" over at her blog A Year of CrockPotting. That post has spawned many comments and other blog posts, with people reporting on results of their various experiments such as using lowfat milk, soy milk, or even coconut milk as starters, different ideas for how to thicken the yogurt, etc.
There are two ingredients - milk and yogurt. (Yes, yogurt - we need the first round of those live cultures to come from somewhere.) Buy plain (as in sugar and flavor free) yogurt from the store for your first batch; use your own yogurt for subsequent batches. Some of the various websites recommend Stonyfield Farm brand for your starter yogurt, claiming it has a higher level of active cultures. That's what I used and had success on my first try so I'll recommend you do the same. As for the milk - I used 2% and got a reasonably thickened yogurt. You can, of course, use whole milk if you don't mind the extra calories.
Incredible Crock-Pot Yogurt
Conceived of by Stephanie at A Year of CrockPotting
Makes approximately 1/2 gallon
1/2 gallon milk
1/2 cup yogurt
Optional - 1 envelope nonfat dry milk powder
Pour milk into Crock-Pot. Cover and heat on low for 2 1/2 hours.
Turn off/unplug Crock-Pot. Let cool for 3 hours.
Remove a small amount of the warmed milk and pour into a bowl. Whisk in the yogurt and return the mixture to the pot. Stir to combine. Optional - stir in envelope of dry milk powder.
Wrap Crock-Pot in a heavy towel and let sit for at least six hours. Longer periods will produce a more dense, tart yogurt.
Refrigerate yogurt to store; it will keep for about two weeks.
Verdict? Amazing. The yogurt was very creamy and mildly tart. None of that artificial aftertaste you get from the store bought stuff. None of the weird gluey consistency or artificially thickened texture. You can tell that this is the real stuff. I'm not going back to store-bought. There's no reason to, what with the fact that this can be made overnight. This is cheaper, super easy, tastes better, uses less plastic, and is kind of fun!
If you prefer sweetened, flavored yogurt, then what is nice about making your own is that you can control exactly what goes in - i.e., no artificial colors or flavors. Go beyond just sugar and try honey or maple syrup. To add fruit flavors just puree the fruit and stir it in. Or even easier - just add a spoonful of your favorite jam to each serving of yogurt and stir it in as you go. The flavors you can make are basically limitless.
And check out the savings: 1/2 gallon of Stonyfield yogurt (2 quart containers) is $12 at my local store. 1/2 gallon of milk is $1.90, an envelope of milk powder is $1, making 1/2 gallon of homemade yogurt more than 4 times cheaper than store-bought. With the completion of my third batch I have now more than made back the $20 I spent to buy the Crock-Pot. Excellent.
Update: I just finished making my third batch of yogurt and still am very happy with it. Using just 2% milk yields a somewhat thin but still quite good yogurt. You have two options for thickening your yogurt: 1) add the packet of milk powder as described in the recipe, or 2) strain out the whey. To do this, line a strainer with cheesecloth and fill with your yogurt. Set the strainer atop a large bowl and refrigerate for several hours. The whey (the sticky liquid that rises to the top of your little yogurt containers) will drain out, leaving you with something like Greek-style yogurt, slightly thicker than sour cream. This has a great mouth feel, very velvety on the tongue. You can also stir back in some of the whey later if you feel the yogurt is too thick. Save the rest of the whey to use in place of water in baking. If you're going to stir in fruit or liquid flavoring, I recommend you use the straining approach to thickening, as the powder-thickened yogurt will begin to thin the more you stir it.