It's available as a Chrome (and maybe Firefox?) extension[0], then websites can use some JavaScript APIs to control it (with user permission of course)[1].

[0]: https://chrome.google.com/webstore/detail/google-cast/boadge... [1]: https://developers.google.com/cast/docs/chrome_sender

On developers.google.com just like you'd expect. :) https://developers.google.com/cast/docs/downloads

I downloaded CastHelloVideo-chrome, put it on my webserver, opened the URL in Chrome, and it worked as advertised. Granted, it does require Chrome and the Google Cast extension, which is as far as I know proprietary.