http://www.runescape.com has been rewritten entirely in WebGL, they just released the beta and are upgrading the entire game in the next month or so.

It's pretty big news, as far as I know they are the first major MMO to swap to WebGL. They were using Java and had been building on it for nearly 10 years but they found that WebGL offered better graphics performance and easier programming than it's Java equivalent so decided to switch. It is slower, probably around 10 or 15%, but offset by all the new graphical features, longer draw distances, more vivid colours, etc.