Twitter is all the rage among SF geeks right now. It's a mobile network similar to
Dodgeball that lets groups of people stay connected via SMS (or IM). The most common use of
Twitter seems to be to blast out to the world (or at least your "followers") what you are doing at any given moment, e.g. "studying for a test," "going to the bank," etc.) For a full write up of how to use it, check out
Rafe Needleman's review.