TogetherJS - TogetherJS is a platform designed by the Mozilla Labs with the feature of real-time collaboration for websites and even applications in addition to watching movies and videos together.
CyTube - Free, open source synchtube
Juicebox.dj - Watch YouTube and discover music online with others.
Metastream - Watch streaming media with friends
Watch2Gether - Watch2Gether is an app that allows multiple people to watch the same content simultaneously and interact with each other while watching.
Rave - Rave is a free iOS and Android app where you can watch Netflix, Disney+, YouTube, Prime Video and more while chatting with friends in perfect sync. It's the ultimate virtual movie theatre!