I know what's available- I have one myself, I just don't ever use it. I don't have a Hulu Plus subscription, nor do I buy music/video through the Play Store. Chrome tab performance is weak at best.
I guess I'm just hoping for a lot more than what I have right now.
That's the reason to be excited about the SDK release, but honestly, regardless of the number of apps even available, I just don't use that many services that I want to stream to my TV. The current crop does pretty well. I would like to see the ability to throw a Flash object to the screen so I can play any embedded video, and I hope to see interesting games and ambient apps, but that's about the extent I can think of.
Chrome tab performance may be weak due to a slow router. I bought a newer Asus and it helped quite a lot. I can play youtube videos within a Chrome tab and it works perfectly well, even with the router several bedrooms away.
With the opening of the SDK, presumably anything - even so far in the comments on this HN posting there are already few individuals who are happy their local media streaming servers can be published, and it wouldn't be too hard to imagine that VLC plugins or something similar are too far behind.
HBO Go and Plex are two apps I am using very frequently with my Chromecast... That gets me both external and locally stored content without a dedicated HTPC to play the media.
legally and in high quality? not really. I wouldn't throw a superbowl party without tv access, but if you want to watch your favorite team when you otherwise can't, its definitely possible.
See http://chromecast.com/apps for the current list.
I really only use Google Play, Netflix, YouTube and tab casting though.