There's an old saying in the manufacturing world: Don't reinvent the dongle. Or maybe it was a wheel or something. In any case, it should be little surprise that Chromecast Audio — Google's low-cost answer to the problem of streaming your music easily all over your home — essentially is on the outside the exact same device as the new Chromecast (erm, Video).
Same circular design. Same microUSB power source. Only instead of a connected HDMI dongle, we've got a couple inches of 3.5mm audio cable. (And in a great yellow color!)
The gist is this: You'll plug Chromecast Audio into a power source, and also into your receiver's aux input, or directly into a speaker itself via the 3.5mm jack. You'll then set it up using the new Chromecast app on your phone or tablet. You can then cast to it like you would a standard Chromecast, except this time it'll just be audio — whether that's from Spotify, Google Play Music, or just a tab you have open on your computer.
If you have multiple Chromecast Audios (and why wouldn't you, right?) you can pick and chose where the music's going. It's maybe not quite as self-contained as a Sonos, and things could get a little complicated if you're plugging into sources that require you to switch to them manually when you want to listen to music. But if you pick up some decent (and low-cost) speakers and dedicate them to Chromecast Audio, this could well be an excellent alternative to much more pricey solutions.