These are websites that I've found useful:

Website Link Description
Sharpwriter.com Link A great starting point with some decent links.
Common Errors in English Link Errors in English? Me? Never! How about you?
Guide to Grammar and Writing Link Another good grammar guide
The Art of Writing Ezine Link Good info and some good links here
Bartleby's Books Online Link Along with some other great books, there's a copy of The Elements of Style by William Strunk. Although it was written in 1918, most of the rules still hold true.