Post-Its for Passers-By – New York Times
From the article:
Socialight leaves virtual Post-it notes, called sticky shadows, in specific sites around the city. A text message pops up when a cellphone is carried into the designated space, which is generally smaller than a city block but larger than an intersection. Started last month in a Chelsea loft by two 2004 graduates of New York University, Socialight now has dotted the metropolitan region with more than 500 stickies.