blockchalk
Ahhh, I like this.
BlockChalk is the voice of your neighborhood. You can use it to talk to anyone, about anything. It’s like a virtual graffiti wall, a community bulletin board, and a poster-covered lightpost, all rolled into one. Using the GPS in your iPhone, it shows you messages that other people have left at the place you’re standing, and it lets you leave your own messages, too. It’s anonymous unless you decide to share more. You can even privately reply to other users while still remaining anonymous.
Simple.