Partyhero - Pass your friends the aux!
Metastream - Watch streaming media with friends
Heartbeat - Listen to music with your friends together, on Spotify
Watch2Gether - Watch2Gether is an app that allows multiple people to watch the same content simultaneously and interact with each other while watching.
TuneMeet - Chat with people who are listening to the same song
StreamParty - StreamParty is a watch communiry, that allows you to watch video streams in sync with the people you care about, on multiple services.