I was going to mention WebRTC! It seems designed for video calling, but there are lots of cool use cases - I recently ran across https://github.com/dmotz/tryste

I was going to mention WebRTC! It seems designed for video calling, but there are lots of cool use cases - I recently ran across https://github.com/dmotz/trystero https://github.com/dmotz/trystero , a dead simple WebRTC library for peer-to-peer multiplayer browser games.