All kinds of information on the English language--etymology of slang words from all over the world, quotes from writers, a collection of "weird words," and many good links. It's a British site, but useful to any English speaker.
Look up words just as you would in a paper dictionary. Most definitions come from the American Heritage Dictionary of the English Language, 4th ed., and usage notes are included. There's also a thesaurus if you need a synonym. This is at the top of my bookmark list.
All about dictionaries, usage books, and style guides, both print and Web-based, with numerous links. The best short writers' reference guide on the Web, created by Jane MacDonald.
A fellow named JohnnyG, who has collected his personal essays on his site, apparently put this up for the public good. If you have any trouble at all with Lay v. Lie, this will set you straight.
Origins of hundreds of common but peculiar expressions like "derring-do" or "on the lam." This link takes you to the archive, which has an alphabetical index. You can order a book on the subject by Evan Morris, the webmaster,
If you write and need critiques from other writers (don't we all?), this is the best place on the Web to get them. Groups for short fiction, novels, non-fiction, and
several other kinds of writing. First-rate moderators keep things civilized.
Paul, a distinguished writer himself, has collected a vast amount of information for writers on this site. It includes a long list of literary markets, print and online; links to online book collections, and lots of other useful data.