While Cam and I were traveling in Europe, we used Amanda's iPod Touch to look for places to stay. In particular, an app named AirBnB.com introduced us to the possibilities of renting rooms in people's homes for a brief time, usually 1-3 nights, at rates far below hotels and guest houses.
We used the app a few times to try and find places to stay in Paris and Berlin, and once in Innsbruck. None of the places we found in the app worked out for us, but that was because we didn't search far enough in advance, and because we didn't have a telephone or regular access to the wireless internet.
I just got an e-mail from AirBnB, looked them up on FaceBook, and went to the web site. There are only two listings in the Keys, one a private room on Sugarloaf and the other a house in Key Colony Beach. It might be a natural for Key West, but the transient rental laws preclude it.