Metastream - Watch streaming media with friends
CyTube - Free, open source synchtube
Kosmi - Digital Hangouts Reinvented
Watch2Gether - Watch2Gether is an app that allows multiple people to watch the same content simultaneously and interact with each other while watching.
StreamParty - StreamParty is a watch communiry, that allows you to watch video streams in sync with the people you care about, on multiple services.
SyncWatch.Video - With SyncWatch you can watch videos and listen to music in sync with your friends. Just create a room and start watching! SyncWatch is free and no login is required.