Here's an idea of mine for free (I came up with this as I was reading your comment, so feel free to critique it):
Instead of uploading a text file, a user can type in an artist and song title on their smartphone. This gets added to a playlist that's displaying on a large monitor/TV/projector screen. This could pick songs from YouTube/iTunes/Winamp etc.
Uses:
- Nightclubs. People can offer track selections for the DJ. These are displayed on the wall. Or maybe there is no DJ at all. Users can vote on their track choice that are coming up next on the screen, as well as offering their own track pick - like HN voting/article submissions. (This would be the geekiest nightclub of all time.)
- House parties. As above.