Grammar and usage resources
Bartleby.com
Without a doubt, the best online reference site on the Web. It has a searchable database of reference guides, encyclopedias, and much more. Just some of the works you'll find here include The American Heritage Dictionary of the English Language, Fowler's Modern English Usage, The Elements of Style, and The American Heritage Book of English Usage.
Guide to grammar and writing
This guide, maintained by Professor Charles Darling of Capital Community College, Hartford, Connecticut, is a comprehensive grammar site with a particularly useful "ask grammar" service.
Yahoo's grammar and usage resource list
Writing sites
Good Documents focuses on creating good business documents in the linked, on-screen environment of intranets and the Internet.
Webgrammar helps with spelling and grammar as well as providing a list of resources on writing, typography, web development, and much more.
Style guides
The Times style guide online
The Guardian style guide online
The US Government Printing Office, Style Manual 2000 online.
Sites that focus on web issues
Content Exchange
Content Exchange bills itself as the "digital marketplace for online writers, editors, and publishers." In addition to putting publishers and writers in touch with each other, it provides quite a good newsletter focusing on web content.
Edit-Work
Edit-Work.com is designed to help online editors and provides some useful tips on creating a style guide, establishing editorial procedures, and so on.