Soundcast [No longer maintained]
Soundcloud. Chromecast. You get the idea.
This project lives in a few places. Github, Google App Engine, and the Chrome App/Extension Store. Google App Engine hosts the receiver page for the second screen.
Must read: http://stackoverflow.com/a/17276475/2474735
HIC SVNT DRACONES: A lot of best practices had to be avoided in order for this to work. Long story short, google doesn't allow the extension background to talk to chromecast. All of this will be changing with the upcoming Second Screen API (~mid to late 2015 probably). For now, this'll do.
https://developer.chrome.com/extensions/messaging#connect https://developer.chrome.com/extensions/pageAction https://developer.chrome.com/extensions/browserAction https://developer.chrome.com/extensions/manifest https://developer.chrome.com/extensions/manifest/storage
This project has a great structure: https://github.com/mailvelope/mailvelope/