Foursquare is a mobile application that makes cities easier to use and more interesting to explore. It is a friend-finder, a social city guide and a game that challenges users to experience new things, and rewards them for doing so. Foursquare lets users "check in" to a place when they're there, tell friends where they are and track the history of where they've been and who they've been there with.
Check in with Gowalla on your phone to stamp your Passport at each place you visit. It's pretty much like stamping your passport in real life.
Facebook announced Places on August 18, 2010. It is a feature that lets users "check in" to Facebook using a mobile device to let a user's friends know where they are at the moment