This article, 150 resources to Help You Write Better, Faster, and More Persuasively has some great links in it.
Sections include resources on Blogs, Legal, Style and Citation, English, Genres, Info, News Digests, Media, Organization (as in organizing your writing), Organizations (as in professional, including a shout-out to SCBWI!), Rhetoric, Tools, Word References and even Writing Software.
There were many I'd heard of, and others I hadn't and thought, "Wow, really? That could be useful..."
A few I bookmarked:
Symbols.com Want to use symbolism in your writing or analyze it in a famous work? Symbols.com can help, with more than 1,600 articles about thousands of signs from Western cultural history. (Try hitting "random symbol" - it's pretty cool.)Check it out and see which ones get the "Wow, that could be useful" reaction from you! (And leave a note in comments about the resources you're most excited to discover.)
ePodunk: ePodunk provides in-depth information about more than 46,000 communities in the U.S. through maps, cemetery listings, and even local newspapers.
and The Arts and Humanities Dictionary (they have DUNE and HARRY POTTER glossaries!)