Unless you're a professional DJ, chances are you've had party guests over that get on your case for your taste in music. We each have our own preferences, and no one wants to be left off the playlist. GrouPlay for iOS ensure they never have to by letting everyone on a Wi-Fi connection contribute.
Setup is simple: one person in a group hosts a playlist and everyone else contributes to it. Users can add songs from their iTunes libraries and the entire group can vote on individual tracks to move them up or down the list. It's a little annoying that you have to add one song at a time (even the hoster can't start off with a pre-created playlist), and it takes a minute or two to add a song to the playlist, but it's a good idea that only needs a few improvements.
This does mean that all participants will need to have the app downloaded, so keep that in mind. For small, recurring gatherings, it would be worth having all your friends download the app.
GrouPlay Social Playlists (free) [iTunes App Store]