Brightkite is a mobile social networking site, that uses GPS enabled devices to tell you about friends close to you, and by close they mean 20 meters, or in your block (200 meters), neighbourhood (2 kilometers), area (4 kilometers) or city (10 kilometers).
Logging into brightkite (there's an iPhone application), you get the chance to “check in”, which posts your location and time and makes it viewable to other brightkite uses in your area. Optionally you can post a note saying what your up to (or post a photograph). You can see other local brightkite members, even if they aren't friends yet – and if your feeling sociable can “nudge” them online and possibly arrange an off line meet.
I need a few more friends with GPS phone for this to be much benfit, but it's a nice idea and one that's likely to become more useful as GPS becomes ubiquitous
