Foursquare The original competition location based app. Check-in to gather badges and fight for mayorship of your favorite hangout spots.
SCVNGR This app is foursquare with challenges. It's a virtual scavenger hunt that gives you points for check-ins and completing missions.
Zombie Attack Zombie Attack uses your GPS to locate you and place virtual zombies on the map around you. Avoid them to stay alive. The app has increasing difficultly levels that include a higher number of faster zombies to keep you on your toes.