Togetherly - Watch Youtube with friends
BuiltWith - Find out the technology behind websites
ShareTube - Watch YouTube, synced with friends.
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
Showgoers - Showgoers is a website dedicated to watching Netflix movies together with friends or loved ones who are not sitting on the same PC.
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.