Foursquare (Free). At press time, Foursquare had pretty much cornered the market on location-based social networking. The service lets you use your mobile device to â€œcheck inâ€ to various locations, earning badges and street cred along the way. Looking for a hip and happening lounge in the ATL? Ask the crowd, then head over and check in. If youâ€™re lucky, you might be declared â€œmayorâ€â€”briefly.