With the release of the Chromecast came the second wave of Google's vision for ultra-simple "flinging" of content from your Android device to a larger display. As the Nexus Q took a belly dive, the much smaller and simpler Chromecast took hold, it being small enough to fit in a person's pocket and able to connect to essentially any Android device nearly instantly. Now that this device is in the hands - or plugged into the HDMI ports - of thousands of users across the globe, Google has begun a push with YouTube to expand support and features.
YouTube has been able to Cast or fling video with several devices for a while. You can do with with a Google TV, you can do it with a Nexus Q, and now you're able to do it with Chromecast. The newest feature to appear (just this week, in fact), is a Chromecast "Cast" button working on 3rd party sites.
While in the past users were only able to fling content from YouTube.com or their Android device's YouTube app, this new feature allows them to fling from embedded YouTube videos from all around the web. This feature is in test mode at the moment - if you see it appear on a YouTube video outside of their main home site, you're amongst the few who have such an ability available.
"With more videos coming to YouTube every minute we’re always experimenting with ways to help people more easily find, watch and share the videos that matter most to them. As always, we’ll consider rolling changes out more broadly based on feedback on these experiments." - YouTube
While YouTube hasn't suggested that this feature is Beta per se, you may want to keep an eye on how it's functioning. This feature will be rolling out to all users based on performance - the comment above comes from a YouTube spokesperson speaking with GigaOM and points toward broad appeal in the near future.