With an existing Foursquare login, users can claim an variety of items as either "Lost" or "Found". If an item matching the submitted description of a lost item is found by someone who has checked into the same location as the original user within the past 5 days, the loser and the finder are both notified.

UPDATE: I kind of had to redo this when the Foursquare api v1 was shut down. It now actually uses OAuth 2.0, which is good for everyone. I never renew my Apple dev account, so you can't get the actuall app anymore, but be sure to check out the new mobile version here:http://bit.ly/4sqlandf. I am still working on the visual design, updating the option names, and adding a "not listed" option where users can submit something a bit more particular.