The Chicago Manual of Style - In my opinion, this is the most comprehensive resource on grammar and usage rules you can buy. You can buy the book at your local bookstore or subscribe to their web site, where you can search and read the entire book along with bonus content. If you are a serious writer, I'd get both. At $30/year ($25 if you catch one of their specials), it is an absolute steal.

Grammar Girl - Grammar's newest superhero and podcast phenomenon, Grammar Girl has made the complexities of English accessible to a wide audience. On top of that, she's made grammar hip.

Guide to Grammar and Style by Jack Lynch - This is a nice site that covers most common grammar and usage issues plus a good number of not-so-common ones. Click the Contents link near the top of the page to skim everything he has, or click any of the individual letters if you already know what you're looking for. Example: for "every day vs. everyday", click the E link.

Easily Confused or Misused Words by Infoplease - Wondering whether it's 'affect' or 'effect'? Visit this page and find out. Infoplease has created a nice, easy-to-skim resource to help you avoid common writing errors. Even if you think you're using everything correctly, go here anyway to make sure.

Thesaurus.com - Searching for the right word? Try here.

Dictionary.com - If you just need to look up a word, this is as simple as it gets. It even includes audio pronunciations for a lot of the entries.

Word Counter - This is a very simple and clever tool that will tell you how many times a particular word appears in your document. Example: It will tell you that you used 'give' ten times, but 'provide' two times. This is helpful for seeing whether you need to use synonyms to make your document more readable.

dictionary.langenberg.com - Here you can find just about any word for any occasion. It bills itself as a Dictionary, Rhyming, Crossword Puzzle, Scrabble, Quotation, Thesaurus, Anagram, and Pig Latin resource.

Funds for Writers - Want to find some markets and make some money with your writing? C. Hope Clark's Funds for Writers is a jewel. Her newsletter is an incredible bargain on top of the great free information on her site.

The Writing Show - The Writing Show is a weekly podcast hosted by Paula B. Her show typically involves interviews or roundtables with writers on a variety of topics. I subscribe to far too many podcasts - most of which I don't listen to with much regularity - but The Writing Show is an exception. It's fun, informative, and a great learning resource for writers.

Triangle Grammar Guide - An entertaining and informative blog that runs in my local newspaper, The News and Observer.

Copyblogger - If you want great ideas for how to improve your copywriting skills, read this blog. It's a gold mine of information in very accessible form.

Language Log - If your friends think you are the world's biggest grammar nerd, you need to read this blog. Otherwise, you may roll your eyes or glaze over entirely. I personally think it's great, which may say something about me.

Computer Acronyms - Ever wonder what acronyms like SMTP, HTML, JPEG, and the like stand for? This site gives you a large dictionary of these acronyms.

Computer and Internet Jargon - Similar to the above (and on the same site), this section explains a variety of computer terms such as what it means to defrag your hard drive, what a motherboard is, what a parallel port does, and - my favorite - what the Blue Screen of Death (BSOD) is.

Wikipedia (The English Edition) - Likely the most comprehensive information resource in the history of humankind. That said, you must remember that just because it's in Wikipedia that doesn't make it gospel. Good, quality research always, always, always includes analysis, verification, and a critical eye.

Infoplease - The online dictionary, Internet Encyclopedia, Atlas, Almanac, and much more. It can be less unwieldy than Wikipedia so if you need to look up something in a hurry, this is a great place to go. It will even help kids with their homework.