This is a cool idea. Anttenna (free) uses GPS to let you find classified listings nearby. It's built on top of Twitter and creates a mobile marketplace for exchanging goods using bite-sized Twitter-style postings they call Anttweeting. You can also chat with people who have items for sale. As always, a service like this depends on lots of people using it, but they seem to be off to a solid start. It's using state-of-the-art social networking to facilitate local exchange. Read more on their website.