Watch2Gether - Watch2Gether is an app that allows multiple people to watch the same content simultaneously and interact with each other while watching.
CyTube - Free, open source synchtube
DJ Clique - Listen or watch your favourite content via Youtube on repeat, shuffle or synchronized on all your computers at the same time. Take turns DJ'ing. Enjoy private or public rooms by yourself or together with your friends in sync.
Metastream - Watch streaming media with friends
TwoSeven - Web application that enables people to watch the same thing at the same time across the globe while...
Sync Video - Ever wanted to watch a video on YouTube or Vimeo simultaneously with your friends, who are not exactly sitting next to you? Now you can watch videos synchronized!