If you haven't heard of kefir, it is a mixture of bacteria and yeasts that grow in milk (or sugar water, see below) and are beneficial to your digestion. The microbes grow to form solid "grains" which can be harvested and used to inoculate fresh batches of milk, they are not really grains, just globs polysaccharides produced by the microbes. The "kefirred" milk is then drunk, either neat or in a smoothie. The taste is like very strong blue cheese with a hint of fizzyness. If you put it in a smoothie with banana you can't even taste it (just we haven't had cheap bananas here since a cyclone knocked out the north QLD banana plantations, hope they re-grow soon, $13/kg means we don't buy them often!). There is a much more detailed explanation on Craving Fresh. Basically, you leave the jar of milk and grains out on the bench for a few days until you start to see some action, then you strain out the grains and use the milk, starting a new batch with the grains and fresh milk. If you have too much keffired milk or want to leave it for a while, you can put the mixture in the fridge, where it will grow more slowing, just needs a milk top-up occasionally.
You can use the milk to drink, or to start lactic fermentation of veges or milk/cream, or to pre-soak cereal grains. Its pretty useful stuff. If you don't like milk, you can also use kefir grains to ferment sugar and water mixtures, I haven't tried it yet, but there's some great recipes on Craving Fresh also.
The great thing about kefir is that it continues to grow, so when you have plenty you can start giving it away. I currently sell mine for $5 for a tablespoon, including postage anywhere in Australia, email me if you're interested eight dot acres dot liz at gmail dot com.
Do you use kefir? Have you tried it? Do you want to?