Cloak

Social networks like Foursquare are great for letting you know when your friends are out on the town, and where they're at. But they're also great for keeping track of where people you don't want to see are at. It's for that reason that Cloak was created. This "anti-social" app leverages location data from both Foursquare and Instagram — with support for more networks promised in the future — to let you know if you're getting too close to someone you'd rather not run into. The best part? It's totally free.