How about a free crowdsourced talking tour guide app.

Basically, something that plots your location (GPS) and which direction you're facing (Magnetometer/Compass) and if there is anything interesting available on the server, play it.

Essentially make it so users can explore areas of interest (say a city) in their own way, instead of a predefined route.

Having a way for locals to upload their own audiotracks so you get the "inside scoop" on everything would be brilliant too.