It's like Friendster all over again: we're being barraged by requests to add people to our Dodgeball friends list. Anyway, Dodgeball is a new service which can tell you which of your friends are within a ten block radius of where you are. Once you've signed up and registered your cellphone number with them, you can "check-in" by sending a text message to Dodgeball with your location (it only works in NYC, Boston, LA, Philadelphia, and San Francisco right now, which is already two more cities than we're likely to be in anytime soon), and then if anyone on your friends list (starting to sound familiar?) has also checked-in and is within range, you both get a text message.
UPDATE: Dennis Crowley, Mr. Dodgeball himself, writes in that right now the beta only works in NYC, and that when you "check-in" not only do your friends get a text message of your whereabouts, but your friends' friends do as well.