I'm a huge geek so I once made a companion app for a Dungeon and Dragons web show called Critical Role. Basically, you use the web app while you watch the show and the app provides additional content. The show broadcasts live on Twitch, so I also made a sort of dashboard mini-app where I could display live content to the app during the broadcast (e.g. polls, fan art, links).
I ended up shelving it after the show's producers basically said, "Be careful about using our intellectual property." I don't blame them, but it was kind of a bummer. I'd love to repurpose the code someday, though.
It's built with MeteorJS, which I don't really use anymore, but definitely sped up the process.