I casted movies from my PC to my Chromecast just fine. Just needed to lower the resolution to 480p, which is fine for 90% of the stuff I watch. Mediahint then lets me stream Hulu and US Netflix through my Chrome browser into my Chromecast. And of course Plex (through the browser, not the app).
Desktop casting is available on the Google Cast plug-in, but it's in Beta at the moment. I personally haven't tested it myself though. I'm sure they'll get it working eventually though.
Rumour right now is that they're working on not just replicating your desktop, but your Android device as well. So good things coming to us Chromecast owners!
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