Some friends hooked us up with some culture for piima last weekend, and I've been having a great time playing with it.  Piima is a culture used in modern Scandinavia to create a buttermilk/yogurt like substance.  The best part is that it works at room temperature, so you don't have to heat the milk, or worry about trying to keep it warm with a yogurt maker, etc. 

You just stir the piima culture into milk or cream and let it stand at room temperature for 24 hours or so.  Cultured in milk, I got something that was maybe a little thicker than cultured buttermilk, but not as firm as yogurt.  I'm in the midst of culturing some cream, which is supposed to come out like thin sour cream, and is also supposed to be good for making cultured butter.  Only time will tell...

There are a number of online sources for piima culture.  Just google for "pima culture" and you'll find several sources. 

One thing to note: once you get it going, it has to be "fed" like kefir grains or sourdough starter.  The piima milk I made earlier in the week was sufficiently tasty (very mild, not sour) that I don't think it'll be a problem at my house.