Sonar cuts through the degrees of social separation by telling you whether you and anyone nearby have mutual connections. Link the app with your foursquare, Twitter, and Facebook accounts, then launch it the next time you're in a crowd. It'll let you know if any of your friends' friends have checked into the same place.

WIRED Check into foursquare spots in-app.

TIRED Doesn't work without a Foursquare account. Small user pool can limit utility.