GeoGraffiti is a geolocation service that centers on little voice notes users can leave all over the world. Before having a native iPhone application it was a voice service you could call into and leave a note that would be associated with whatever ZIP code or telephone number entered through your phone's keypad. It wasn't the most exact system, and to remedy that parent company Slingpost has been working on a native iPhone application that makes use of the handset's GPS to make the voice notes a little more precise.
In its pocket form GeoGraffiti is now quite a bit smarter. You can use it to browse "voicemarks," which are other users' recorded messages from wherever you are. It doesn't say how close or far the radius is, but I'm assuming it's only about one to two miles--if that. Since I'm in the middle of San Francisco I found two nearby voicemarks, both with fairly precise geographic information, including addresses. These are the same voicemarks that have been created on phones as well as on an accompanying Google Maps mashup. The app does not make the distinction as to which platform they originate from.
The app will constantly ping for your location, and if you're on the move it will update every few minutes. If you're driving your car, you can set it to "hands free" mode, which will call your phone and transmit any new, local voicemarks via a telephone call. It would be nice if you could just have it funnel through the app to save voice minutes, but it's almost like a radar detector for messages.
This app reminds me of some of the local note applications that I've seen getting buzz on the app store (see Graffitio, NearPics, and Zintin). There's a certain human fascination with scribbled graffiti and notes written in guest books. This is a smart play on that. I wouldn't be surprised to see people take advantage of something like this to create their own scavenger hunt or to leave messages for friends, making it a platform of its own. Adding some things like special password-protected drop boxes or easter eggs would definitely take it to the next level.
To see a demo of the service in action I've embedded an early look below. Just note it was produced before the creators got their hands on the new hardware.