Dictionary.com Every writer needs a dictionary – we just also happen to be fond of this online one. Fast and cheap.
Thesaurus.com is occasionally plagued by popup ads, but it’s the most thorough thesaurus out there. Will find synonyms for even outlandish words.
Big Dog’s Grammar is simple enough that it may frustrate more technically proficient authors, but its non-technical word choice and straightforward approach may be particularly helpful to young authors or authors who speak English as a second language. The site has a decent sense of humor and is heavy on examples.
OWL is Purdue University’s Online Writing Lab. It is clear enough to help someone with almost no exposure to grammar, but comprehensive enough to be useful to a writer with a strong grammar background. Includes printer-friendly quizzes.
The University of Calgary grammar guide focuses on defining sentence elements and parts of speech but includes sections on plural v. possessive, improper word use, overuse of euphemism, and subject-verb agreement. Site employs frames and features quick automated quizzes within each section. May be more useful as a dictionary than as a teaching tool.
The King’s English, published by H.W. Fowler in 1908, still feels modern thanks to its emphasis on clarity and simplicity of language. It contains an extensive section on common malapropisms, but its main emphasis is on style choices which will stand the test of time. Relatively brief.
The American Heritage Book of English Usage deserves to be worshipped by any dedicated grammarian. This book goes well beyond basic grammar rules and into the arenas of style and current usage. It includes sections on gendered language, science terms, and e-mail conventions. Extensive. (Also our Associate Editor’s favorite website.)