Built using node.js, sockets and heroku.
Unfortunately, with the current state of online music store terms of services, it is unlikely that this is the case for the majority of music in most people's collections.
It's got a really easy API, though, so you can build other clients without too much trouble.
(feel free to translate into other languages)
A war with a 500 watt amp driving it.
On this note, I wish Sonos had an API. You can hack in UPNP commands to it though, so that might work.
