4. Swarm
Platform: iOS and Android
Price: FREE
Foursquare has joined the app pack by releasing a fast way to meet up with friends: Swarm. Find friends near or far away from you based on miles and share what’s going on, or message them with a game plan for fun. When locating others on the service, it also passively notes your general location even if it’s not directly opened in the app. So if the app downloader is in a subway station and someone looks at their phones, Foursquare will understand that their in a new neighborhood, and update the status.
 "Pin It")