The Chromecast is a stripped-down ChromeOS powered dongle that lets you share media content from your Android, iOS or Chrome browsered devices to your TV. Oh, and it's $35. Yes, $35.
The best way to explain the USB powered Chromecast dongle is that it's another attempt at the Nexus Q formula but much cheaper, with the developer APIs that should have been there for it's 'predecessor' and powered by Chrome OS instead of of Android.
The premise is this - you find some content you want to play on your device in a Chromecast supported application (e.g YouTube, Play TV & Movies, Play Music, Netflix etc.) and then hit the button in said app to send it to the Chromecast dongle that is plugged into your TV's HDMI port. The content isn't streamed across your network - as with the Q the content is pulled from the Internet directly by the dongle so you can carry on doing other things with your device.
Pretty neat? I think so.
There is actually a chance that the thing could take off this time. Not only is it ridiculously cheap, but Google are making available (in Beta initially) APIs to allow Android, iOS and Chrome developers to integrate Chromecast functionality. This means that iPlayer, CatchupTV, Sky Go etc. support isn't beyond the realms of possibility. OK, Sky Go probably is, but that's just Sky. ;)
Another Beta feature is the ability to send contents of a Chrome browser tab on your PC to the Chromecast device. Great for photos etc. and maybe a bit of browsing, but probably not performant enough for other streaming video? We'll wait and see.
For now the Chromecast dongle will only be available in the US (from the 28th July), but Google say they are bringing it to other countries (including the UK) 'as soon as they can'.
I can't wait! :)
Check out the Chromecast page on Google Play for more details.
[EDIT: Shipping from Amazon US immediately!]