Passing Glances [via NotSoSoft] starts as a forum for people who pass each other on the street to get in touch. See also i saw you: send "zonagrams" (ugh) to people who caught your eye. Basically a location-based message board. (Nice idea, but not well implemented, and suffers heavily from scaling problems: works well if everyone in an area takes part, and progressively less well the fewer people use it. Obviously going to be very bad to start off with then, so no incentive for people to start using it. Therefore will never take off.)
Easier: Take a photo of your stalkee and have it printed on a postcard with your (temporary) email address at the bottom. Drop the card on the pavement just before your victim comes along, and hope they pick it up. Or have the picture printed on a t-shirt with "Talk to me, I'm desperate" in big writing underneath.