Laserquest + smartphone => fun

Imagine playing laserquest (aka Quasar) anywhere, with anyone, just using your phone. Play on the street, urban warfare style; play in the park. The necessary additions to existing smartphone technology are a transmitter at the top of the phone and a set of sensors on its sides. The phone can be held like a StarTrek phaser or clipped into a gun (like a Wii-mote fits into a plastic gun).

This concept makes the best use of smartphone technology: the screen displays a map, and puts you and your opponents on as (labelled, different coloured) dots (using GPS technology); you can call you friends whilst playing to coordinate tactics (a bluetooth headset is preferable); flip the phone's camera forward to video (or take stills of) the action (for upload via 3G); automatically log scores online (using 3G) to a national competition database or just a score table of you and your mates.