Whisper: A GeoLocation based Communication service.
Whisper allows you to broadcast messages from the location you are standing. With these messages, you can specify a loudness. The louder your message, the greater the area it can be read in, and the quicker the message goes away.
Want to play a game with your friends? Give them clues and leave small whisper messages directing them to the next location.
Messages automatically appear and disappear when you leave a location or the messages get too old.