Dennis Crowley, Naveen Selvadurai and their tiny team of 20-to-early-30-somethings have created one of the most exciting mobile-phone applications across the two coasts: FourSquare. The app, part friend-finder, part social city guide and part nightlife game, allows users to “check in” to restaurants, stores, cafes, parks—all kinds of locations—and let their friends know where to find them and what kind of fun activities they can do. Businesses have just started signing on to reward FourSquare users with incentives to “check-in” to their spots, even offering discounts to the “mayors,” or the most frequent visitors to their locations. Since debuting in March, FourSquare has launched social guides in dozens of cities all over the world, and the iPhone app has attracted more than 150,000 downloads.