Mobzombies is a game I made with Aaron Meyers that started as a tablet based GPS game and has been ported into something you can play on your phone.
The original version used a custom sensor rig that mapped each step you took in the real world to a step in the zombie world (At the time we made this mobile/location based games were pretty novel). We showed the game off at the (now defunct) Wired Nextfest in 2007 and the Maker Faire. The game is a mix of GPS, augmented reality, and a classic zombie shooter.