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.
Editor's note: This article was commissioned and submitted by Mitchell Waite, the publisher of iBird Explorer and CEO of Mitch Waite Group. Laura Kammermeier is a respected voice in the field of birding and has previously published on the subject of iPhone apps. We normally don't encourage submissions from app developers, when they concern apps or accessories they develop.