OpenSync is a free, open source, and cross platform site for synchronizing the playback of YouTube videos. It is currently in early development but is completely functional. It is available online at https://opensync.live or you can clone the source and run it like you would any dotnet core application, see self-hosting.
You'll need the following to host OpenSync:
- .NET Core >= 3.0
- A Google API Key
Once you have your API key, set it as an environment variable called googleapikey and that's it!