Alternative Dictionaries contains words and expressions you most likely won't find in a normal dictionary, such as slang.

John Bartlett, Familiar Quotations

BBC Online, a wonderful listing of dictionary and word sites. Thanks to my friend Charles Christ for suggesting this link to me!

Biotech Life Science Dictionary

A Collection of Word Oddities and Trivia

Dictionary.com

Dictionaries & Encyclopedias

DOT - Dictionary of Occupational Titles

Focusing on Words (focusing on the Latin and Greek elements used in English)

Jessie's Word of the Day

List of Dictionaries

Literary Dictionary (words that first made an appearance in books, plays, poems, comic strips, and mythology)

Merriam Webster's Word of the Day

OneLook Dictionaries

Phrase Finder (give it word, and it'll give you a list of phrases that are related to the word in some way

Quotation Page

Roget's Thesaurus

Semantic Rhyming Dictionary

Skeptic's Dictionary(reference book with over 200 entries on the paranormal, the occult, the supernatural and the pseudoscientific

Syndicate.com (increase vocabulary skills)

travlang's Word of the Day

Valuable World-Wide Word Links

Visual Thesaurus

A Web of Online Dictionaries (over 330 dictionaries for more than 100 languages!)

Wordbot

Word Detective

WordNet(powerful lexical reference system that combines aspects of dictionaries and thesauri with current psycholinguistic theories of human lexical memory)

Word Play (tons of links for anyone interested in words)

Wordsmith.Org (A Word A Day, Internet Anagram Server, Word Server, Listat)

WordWatch (commentary on current English)

Word Wizard is for those folks who are fascinated with words. They offer a selection of new words, snappy quotes, elegant insults, competitions, Public Scribe Service, Fancy Word Parties, and Lexicographer's Club.

World Wide Words(articles about words, word searches, usage notes, and more)

WWWebster's Dictionary

Languages and Slang:

American Slanguages is a great site where you can learn to talk like the natives of a particular area. Great spot for getting your dialogue right!

Educational Pet Rock is a site to help you learn foreign languages . . . the *fun* way!

The Ethnologue is a catalog of the world's languages including information on alternate names, number of speakers, location, dialects, linguistic affiliation, and other sociolinguistic and demographic information.

Language Identifier

Non-Sexist Language

Travlang's Online Translating Dictionaries

Twists, Slugs and Roscoes: A Glossary of Hardboiled Slang will help you translate "slang" into English.