iOS: We all have a few people we're friends with on social networks who we just don't want to ever see in real life. Cloak is an app that makes avoiding those people a little easier.
Cloak connects with Foursquare and Instagram (with more social networks coming in the future) and shows you a map of recently updated locations. You can just glance at the map if you want, but you can also flag certain people and receive an alert when they get near you. That's your cue to hide behind the trash can or duck into the nearest store.
Cloak's a bit silly, but if you're desperately trying to avoid your ex or you have an uncanny knack of running into your boss when you're day drunk, it could be a lifesaver.