VideoJS - VideoJS is an HTML5 Video Player, built with Javascript and CSS, with a fallback to a Flash video...
Metastream - Watch streaming media with friends
Plyr - A simple, accessible and customisable HTML5 media player
Watch2Gether - Watch2Gether is an app that allows multiple people to watch the same content simultaneously and interact with each other while watching.
Flowplayer - Flowplayer is a video player for web sites.
Hyperbeam - A better way to watch together online