PseudoDictionary

Covers slang, made up words, "webspeak", colloquialisms and accepts contributions for new entries.
http://www.pseudodictionary.com/

The Online Slang Dictionary

A global dictionary of English slang.
http://www.OCF.Berkeley.EDU/~wrader/slang/

Carleton County Colloquialisms

Dictionary of the colloquial language of Carleton County, New Brunswick, Canada and Aroostook County, Maine, USA. Audio clips accompany each entry.
http://dooryard.ca/

A Dictionary of Slang

English slang and colloquialisms currently used in the UK.
http://www.peevish.co.uk/slang/

London Slang

Online word and phrase dictionary of Cockney rhyming slang. Also includes examples of film slang.
http://www.londonslang.com/

Slang Search

Definitions of common words in English-language slang, organized by subject area.
http://www.slangsearch.com/

Sayings Explained

Searchable database of well-known sayings, phrases, and slang, along with definitions and origins.
http://brochuresonline.net/sayings/

Insultmonger.com

An alphabetized compendium of expressions considered offensive, vituperative, and insulting.
http://www.insultmonger.com/slang/index.htm

The New American Sub-Standard Politically Incorrect Dictionary

A glossary of offensive terms or terms with offensive meanings.
http://www.americansubstandard.com/

Businessballs.com - Cockney Rhyming Slang

Origins, derivations and dictionary translations. Cockney rhyming slang uses substitute words, usually two, as a coded alternative for another word.
http://www.businessballs.com/cockney.htm

Urban Dictionary

U.S. slang dictionary whose submissions and various definitions are subject to votes for inclusion.
http://www.urbandictionary.com/

SlangSite.com

A dictionary of slang, webspeak, made up words, and colloquialisms. Submissions welcome.
http://www.slangsite.com/

Chinese-English Slang Translations

Various expressions, usually colloquialisms or slang, translated for the benefit of native Chinese speakers learning English.
http://www.angelfire.com/space/googleyahoo/files/slang.htm

Double-Tongued Word Wrester

A growing dictionary of words from the fringes of English, including jargon, slang, and loan words.
http://www.doubletongued.org/