Slush: Paula Guran's report from the trenches. Interesting!
The Write Page: Electronic Newsletter for writers of genre fiction, with many links to authors' pages.
Strunk & White: The Elements of Style is the single most important book a writer can read. Read it carefully and return to it often; you'll be amazed at the wisdom it contains. This is an electronic version of Professor Strunk's original text. White's major addition is a chapter on style, but the basics are all here.
Grammar and Style Notes: Not sure if you want a semicolon or a comma? Check here. An excellent collection of advice on grammar and style.
SFWA: The homepage of the Science
Fiction Writers of America. All sorts of very good advice, and lots of
links.