GrammarBook.com provides some grammar and punctuation rules, as well as a portal for ordering The Blue Book of Grammar and Punctuation.
Ask the Grammar Lady is a free question and answer service provided (via e-mail) by an English teacher.
A Web of On-Line
Grammars has links to online grammars of as many languages as can be
found on the Web. It includes all types of grammars: reference grammars, learning
grammars, and historical grammars.
Common Errors in English is a Web site dedicated to "dedicated to errors in usage" (e.g., accept vs. except) that was created by an English professor.
World Wide Web Consortium (W3C) is the official resource for Web technologies and standards (e.g., HTML standards).
Publishing, Copyrights, and Trademarks
Copyright Clearance Center is a not-for-profit
organization created at the suggestion of Congress to help organizations comply
with U.S. copyright law.