I'm sure there are a lot of contrived examples, but are there any good ones? Facebook and Twitter.
Ok, but are there any good and common ones?
I'm failing to see the advantage of reverse http over long polls, even though I am completely sold on the difference that having server push would make...
The only reason I hadn't use xmlhttprequests extensively before it became popular (and renamed Ajax) was because it was too much work, too error prone. The popularization of Ajax lead to solid frameworks and libraries, which fixed that.
Or a bug tracking system. Or a newspaper. Or pretty much any site, really.