Location based social-networking is a concept that has been bouncing around the net for a while now. The idea is to make getting together and interacting with people easier by sorting users by--or simply displaying--their geographic proximity to you. Services such as dodgeball have become hugely successful in mobile markets, and Meetro is attempting to do the same thing for desktops and laptops. With Meetro, you create a buddy list, and are alerted whenever someone on that list comes within a user specified distance of your current location. I'd get into the magic internet voodoo that Meetro uses to figure out where you are, but their FAQ page is currently down.
Although Meetro is the first location-aware instant messaging service that I know of, I can't see it taking off. My issues with it are two fold; I have to create a whole new identity to use the service, and if I want to know where my friends are, I have to get them to do the same. Although the interface of the program has come a long way from when I first saw it in its early beta stages, its still not nearly as user friendly or intuitive as I would like. Ideally, I'd like to see a plug-in for iChat or Adium which allows me to see the location of my regular AIM/MSNIM/YIM buddies when I mouse over them.
In any case, Meetro is certainly an interesting concept, and worth checking out, even if only to get a taste of what surely will be many similar services to come.