Many of the features will look familiar to people who have set up YahooGroups before, but there are a number of features that take advantage of geograhpic proximity information. Definitely worth a look if you'd like to create more social connections *or* more political activity in your neighborhood. Even has a carpool finding feature, though it doesn't try to real-time matching.
Keith developed this software for use in field studies of four Boston neighborhoods. He hasn't written up the results yet, at least not for public consumption, but he describes some of the results in an interview. Sounds pretty amazing!