Thesaurus.com - This clever, clickable resource stays open in my browser all the time. (Beats flipping through a paperback alphabetically. ugh) It also has a dictionary.
The Bookshelf Muse (blog) - [*Note: The Bookshelf Muse is now Writers Helping Writers.] This is an awesome site run by Angela Ackerman & Becca Puglisi. You should check out their writers' thesaurus' - divided by categories like emotion, setting & weather. They also have craft books available for sale. Superb resource!
Grammar Girl - Quick and Dirty Tips - This is grammar with a side of humor. [*Note: Though I love this site, it has become rife with advertising. Proceed at your own ad and pop-up risk.]
Grammarbook.com - I like this one, too, mainly for it's wealth of examples. If you need it spelled out for you, this is the one.
Notorious Confusables - This is a list of commonly-confused words, such as affect & effect or allude & elude. I've linked you to the 'choose from the alphabetized list' page, but on it are links to part 1 and part 2, for those who want to peruse them all.
Random Sentence Generator - It took me a while to see a use for this, but now I do. The idea is not to copy the phrase, but to let the sentences and phrases pry your brain out of a rut and spark some creativity. If you're blocked or having a slow writing day, go here and click the button a few times. You'd be surprised what an inspiration this can be.
Character name generators : Random Name Generator & Fantasy Name Generator
If you're new to the jargon and the craft, you love this Writer's Glossary. It covers everything from genre to query to POV.
Cliche Finder - Haven't played much with it yet, but I can see a few uses for this one. It could let you see if something in your work is cliche, or it could help you find some good ones for a character who speaks that way. For example, here's the results for a search of 'ugly.'
Dialogue Tutorial: Star Wars version - Need help formatting your characters' dialogue? I can't promise this follows Chicago Manual of Style, but it's fun. = )
Books on the Craft
Self-Editing For Fiction Writers by Browne & King
(Don't let the title fool you - this one deals with far more than just spelling and grammar.)
Techniques of the Selling Writer by Dwight Swain
Plot & Structure by James Scott Bell
The First Five Pages by Noah Lukeman
(This one may make you want to self-publish *gasp*, but it will save you time and trouble if you decide to pursue a traditional publisher.)
Beginnings, Middles & Ends by Nancy Kress
Show & Tell In A Nutshell by Jessica Bell
This is an excellent resource for beginning fiction writers. The author gives several examples of passages written in bland telling form followed by the same scene once showing techniques have been applied.
Showing & Telling: How to Show & When to Tell for Powerful & Balanced Writing by Laurie Alberts