If you don't know what a Chromecast is or have not paid attention to the details, (many people have very misguided notions about how it works), see these earlier overview articles.
These are the latest with new information:
HBO GO, Revision 3, Washington Post, Songza and more currently being tested for Chromecast
Chromecast teardown reveals a simple, easy to disassemble device
The battle between Android and Chrome is over -- and Chrome won
Keeping it simple: Chromecast, Google TV, and the zen of a $35 dongle
How to Stream Local Files Using Google Chromecast
Chromecast news: Google releases open source code, hackers working on root
Paul Thurrott (Windows pundit) who famously labeled the iPad an "iDud", has this to say about Chromecast:
If you feel like you’ve read this story before, you have: Apple does the same thing with AirPlay and its Apple TV, as does Microsoft with Play To/Play on Xbox/Smart Glass and the Xbox 360. Will Google’s third try succeed? No, because no one has found the secret sauce to the living room yet, and this me-too solution certainly isn’t different enough to be a big deal. Sorry, Google, but Roku already works just fine, thanks.He is amazingly wrong on so many levels. First of all it works very differently than a Roku. It is much faster and simpler than a Roku. While Apple and Microsoft do have similar technology, they both keep it tightly controlled. Apple and Microsoft also charge much more - which matters a lot when you want to support multiple TVs. It is easy to make apps that take advantage of Chromecast. Google already supports it from Chrome (including Linux, MacOS, Windows and ChromeOS), iOS and Android, neither Apple nor Microsoft come anywhere close to supporting that number of devices. In it's breathtakingly simple ease-of-use, the Chromecast is anything but a "me-too" solution. It is to TV boxes, what iPhone was to earlier smartphones. In fairness to Paul, he's far from the only one to misunderstand the device. What really amazes me is how many do not understand that its secret to success is its cross-platform capabilities.