A. Online Dictionaries    
Language, spelling, definition, and translation tools.

Bartleyby.com - english.. american heritage dictionary and other tools for writers.
Cambridge Dictionaries - english.. search dictionaries, idioms, and phrasal verbs.
Dictionary.com - vocabulary.. online directory of dictionaries and other references.
Merriam-Webster - english.. collegiate dictionary and thesaurus.
MSN Encarta - english.. north american edition.
Newbury House Dictionary - english.. definitions, photos, sample sentences, and more.
yourDictionary.com - vocabulary.. multi-lingual and specialty dictionaries and more.

B. Specialty Dictionaries 
Useful language tools and dictionaries dedicated to specific subjects.

AcronymFinder - acronym.. directory of acronyms, abbreviations and initialisms.
ClicheSite.com - cliches.. clichés, sayings, euphamisms, phrases and figures of speech.
Foreignword.com - translator.. contains 56 source languages and 58 target languages.
Glossarist.com - glossary.. compendium of glossaries on various subjects.
InvestorWords - business.. alphabetical index of investment and business terms.
RhymeZone - rhyming.. enter a word for rhymes, phrases, synonyms, more.
SpellWeb - spelling.. search engines vote on the spelling or popularity words or phrases.
Thesaurus.com - thesaurus.. a treasury of related and contrasting words.
Webopedia - technical.. computer, internet and technology terms.
WordCentral - word fun.. access student dictionary or build your own; create a rhyme