We released a feature called Collaborative Playlists yesterday, which basically is a playlist who anyone subscribing to it can modify.
It’s pretty simple, create a new playlist, option-click it and select “Collaborative Playlist”. Add some songs if you wish and send the link to a few friends. (you can drag the playlist into an instant messenger, a e-mail message, or simply option-click the playlist and select “Copy Link”)
There is a good article in our official Spotify blog called Collaborative playlists which I recommend you have a look at.
Paul Lamere has written a pretty good article too and posted one of his collaborative playlists, right here: Collaborative Playlists
Here’s my shared, collaborative playlists: Morning Glory - Click the link, add something you see might fit and let’s see what happens!