Every writer should have a basic set of language usage tools. Bookmark these three sites for the next time you’re scratching your head, wondering, “which or that?”:
Garbl’s Writing Center: http://garbl.home.comcast.net/~garbl/
A wealth of links. Writing Resources alone has links for Creativity Resources, Writing Process, Grammar Guides, Style and Usage, Fat-Free Writing, Plain Language, Action Writing and others. The last link, Word Play, is full of fun stuff.
The editorial Style Manual is akin to an AP Stylebook.
The Concise Writing Guide provides examples of how to trim down wordiness.
The Bookshelf of Writing References has book recommendations for everything from grammar to language usage to creative writing to quotations to advice for web writers.
WordWeb Online: http://www.wordwebonline.com/
Your basic dictionary. Encyclopedia links back to Wikipedia (word of warning: Wikipedia can be skewed, and in some instances, dead wrong. Case in point: Someone had uploaded a “story” that Pennsylvania Governor Ed Rendell had died. Wikipedia didn’t catch it until a Rendell staffer reported that rumors of his death had been greatly exaggerated.)
University of Alabama at Birmingham’s Writing Style Standards A to Z: http://main.uab.edu/show.asp?durki=7713
Geared toward journalists, but useful for all writers. “The UAB Guide to Style and Design provides guidelines and examples to help maintain consistency in written materials. It is based on three common style guides: the Associated Press (AP) Stylebook, The Chicago Manual of Style, and the American Medical Association (AMA) stylebook.”