TrackDropper for Android combines geocaching and music 'piracy'
Like geocaching, TrackDropper lets you leave something in a place that you've visited -- but instead of a keepsake in a Tupperware box, you leave digital tracks in a virtual space. Other TrackDropper users can then visit the location of your musical 'booty' and listen to it -- and leave another song in its place! There's a cute video of it in action after the break.
Unfortunately, I can't get TrackDropper to work on my phone -- it might be for Android 2.1 or later -- but I don't think it works by uploading MP3s to a centralised server. Instead, you store the unique ID of the song which, when someone finds it, is then played back using the 7digital API. Of course, should Google or Apple introduce a similar service (and they surely will), they will have their own streaming APIs in place.
Incidentally, TrackDropper was one of the many hacks output by Music Hack Day London -- if TrackDropper isn't your kind of thing, check the huge list of other hacks.