Metastream - Watch streaming media with friends
CyTube - Free, open source synchtube
StreamParty - StreamParty is a watch communiry, that allows you to watch video streams in sync with the people you care about, on multiple services.
WatchParty.me - WatchParty.me is a video streaming platform where you can watch streams with your friends in a private virtual lobby.
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.
Netflix Party - Netflix Party is an extension for watching Netflix remotely with friends and family for movies nights with that long-distance special someone.