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/addon/spdy-indicato...
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.
To sum it up, clients (SPDY enabled by default in FF13 and already in Chrome) and servers (Apache, nginx, Jetty, node, Google sites) are becoming available, some niches (mobile, high latency) are bound to benefit a lot and it's a good (better?) solution for the general case.