As of today, the ability to cast content to your Chromecast from the desktop comes in the form of a Chrome extension called Google Cast. As of the latest beta build of Chrome, though, you can now cast content from the web without said extension installed. All you have to do is right-click the content and use the “Cast…” menu…
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This comes via François Beaufort, who mentioned the new feature on his Google+ profile. To see the feature, all you have to do is enable the experimental flag “Media Router” at chrome://flags/#media-router. From there, the Google Cast extension (if installed) will be disabled, and see a new “Cast…” menu when you right click the page.
This is only in the beta channel of Google Chrome for now, but chances are that it will make its way to your installation of the stable build sometime in the very near future.