You can visit, for example, a TED talk, hit the bookmarklet and it will pull down the MP4 behind the scenes and present it on a layer that's now Chromecast friendly. Same goes for Vimeo, Plex.tv (without having to pay the $5/mo) and quite a few other video sites that are most likely against the TOS to link to.
Definitely open to hearing the thoughts of the community!
Question: is this 1) transcoding like "cast this tab" does (assuming not), 2) routing the original data stream via my computer, or 3) having the Chromecast download the stream directly?
And if #3, are there any issues with login permissions, cookies, etc. not being present on the Chromecast?
I will definitely try it out with plex later today.
Plex is awesome, though, and they deserve that $5/mo for making the Chromecast actually useable.