This site is a big long, but it's the first hit that has a guide- http://allaboutchromecast.com/chromecast-how-to-guide/how-to-use-chromecast-to-share-desktop-screen-and-audio-to-tv/

Basically, install the Cast extension on Chrome, click the down arrow and choose to cast your whole screen (rather than a current tab).

The AppleTV will let you do this. I don't know if you can use cast to extend your desktop.

This says it can. Just type "second display" into Google play and browse through them. I see at least two that say they can do it.