Now added to the small list of SPDY enabled sites as I find them: https://www.webtide.com/ https://twitter.com https://spdytest.com https://spdy-twitlog.indutny.com https://www.codecentric.de (and of course google.com, gmail and youtube.com) Of course the number will explode in a few months but fun to know the handful in the "early days". ps. SPDY indicator for Firefox 11+ https://addons.mozilla.org/en-US/firefox/add…
Sadly it looks like supporting SPDY for Firefox has to be special-cased (the indicator checks for X-Firefox-Spdy: in the headers). Because of this only a couple of the above servers support it.
Unfortunately firefox doesn't seem to currently set any kind of accessible variable to indicate SPDY so I suggested an idea to detect that injected header for a Firefox port of the chrome indicator extension. Cheng Sun made it an hour later.