Dictionary.com allows you to search multiple dictionaries and references at the same time.

Dictionary.net returns word and phrase definitions from a variety of English dictionary resources.

iTools has links to a number of dictionaries, thesauruses, and other language tools, including online translators.

The Merriam Webster Dictionary allows you to search this dictionary site from your computer. Search options also include Spanish-English, thesaurus, and medical terms.

LookWAYup Dictionary/Thesaurus and Translation is a large online English dictionary that tolerates small spelling errors (U.S. or international spelling), shows definitions for multiple senses of the word, and for each sense shows synonyms, related terms, and usage examples. It will also translate words to French, Spanish, Portuguese, Dutch, or German or vice-versa.

YourDictionary is a free resource that simultaneously provides dictionary, thesaurus, and etymological references as well as definitions for scientific and business-related terms.

A searchable list of biographies are available at S9 the biography dictionary.

The Visual Thesaurus offers a visual way of exploring the relationships between words.

Bartlett's Familiar Quotations searches familiar and famous quotations.

The New York Times on the Web: The Glossary of Financial and Business Terms contains a list of business terms from A to Z.

William Strunk's original version of the Elements of Style (later made famous by E. B. White) is available courtesy of the Bartleby project.

Robert Harris has a searchable rhetorical devices handbook and a glossary of literary terms.

WebGrammar is a site devoted to grammar.

The Apostrophe Protection Society is a good-natured site that promotes proper use of the apostrophe.

iScribe is a site devoted to grammar basics and definitions of grammatical terms.