I've wanted to program the music on the radio for a while. Now, in a way, I can! It's called Blip.fm.
Each user, known as a 'DJ', has their own station; which is a place to post the songs that they have picked. The library of music to choose from is huge (and ever-growing)! I think it's mostly sourced from track previews from iTunes and Amazon. Choices are beamed out, through services like Twitter, or Facebook. The music you choose to play appears on your station.
DJs find each other because of their tastes in songs or music. They can applaud good selections (by giving props to that person), and then add that song to their own playlist.
It's so cool to have this treasure chest of music to browse... and then choose from. I can simply pick a song and know it will (very likely) be there! When I find something I like, I can post it in real-time.