First off is ShadowPriest.com this is the holy grail (so to speak) of websites for Spriests. It has everything from a gear listing, to rotation/spell priority discussions, to player gear checks and talent spec suggestions. This site really has it all (almost) when it comes to Spriests.
The next most useful site that I have found recently is Ensidia's website. Being one of the top guilds in the world, they have some extremely helpful information when it comes to any class, but Muqq and Eoy offer some incredible insight into the Spriest, and have detailed guides for specific fights. Their site is is open for registration and posting, and both Muqq and Eoy do reply to posts on topics (as long as the topic is reasonable).
Fear.Win has an interesting take on the Shadow Priest A-Z. While the A-Z list is sort of gimmicky, it does have some very useful information in the list.
Misery often discusses many aspects of the Shadow Priest, not unlike what this blog is intended to do, but it has been around for much longer, so it is definitely worth delving into.
Those are the main resources that I have personally located regarding Spriests, but I am sure that there are many more out there that I have missed. Please feel free to link any other useful Shadow Priest related sites that I may have missed.
See you in the Shadows,