It's not often (in fact it's never) that you see a game tagline line this: "Make music with a disc of 510 067 420 km² = the Earth." But that's what Oterp, a hombrew PSP app by Antonin Fourneau does.
Oterp uses the PSP GPS add-on to track your relative position on the planet and alters music in real time based on your movements. Its developer has also hard-coded numerous "collection" spots around the globe into the software. Travelling to these locations will unlock new music and effects, similar to unlocking characters via GPS in the Japanese release of Metal Gear Solid: Portable Ops. Sounds pretty cool, huh? More videos of it in action can be found at the Oterp site.