With Foursquare, it's all about community. Let Foursquare know you're at your favorite pizza place and the app will load tips from other users, or alert you to other Foursquarers currently in the joint. Check in to a location often enough and you're dubbed the "mayor," an honor that sometimes comes with discounts. Location-based social media is currently all the rage, with players such as Gowalla and Facebook Places also competing for users. But with five million registered users, Foursquare still dominates.