Slang

Businessballs.com - Cockney Rhyming Slang
Origins, derivations and dictionary translations of the slang used by inhabitants of the East End district of London, England.
http://www.businessballs.com/cockney.htm

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 Old Hobo Slang
A searchable compilation of slang used by vagrants and migratory workers, listed in alphabetical order.
http://www.hobonickels.org/alpert04.htm

A Dictionary of Slang
English slang and colloquialisms currently used in the United Kingdom. Includes definitions, a part of speech listing and an example sentence for each term.
http://www.peevish.co.uk/slang/

Double-Tongued Dictionary
A growing dictionary of words from the fringes of English, including jargon, slang, and loan words. Browsable by category or searchable by letter. Includes analysis of new words, and modern connotations of words with old denotations.
http://www.doubletongued.org/

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

The New American Sub-Standard Politically Incorrect Dictionary
A glossary of offensive terms or terms with offensive meanings. Searchable by letter and browsable by the topics "Cool Evil Words" and "Funky Proper Words." Submissions accepted.
http://www.americansubstandard.com/

The Online Slang Dictionary
A global dictionary of English slang compiled from user submissions. Includes a slang thesaurus and word lists browsable by letter.
http://onlineslangdictionary.com/

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

Sayings Explained
Searchable database of idiomatic phrases and rhyming slang, along with definitions and origins.
http://brochuresonline.net/sayings/

Slang O' The Day
English slang words with origins from various countries around the world including America, Canada, Australia, New Zealand, Ireland, and Wales. Updated daily and includes definitions and example sentences.
http://www.slang.otheday.com/

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

Slangasaurus - The Slang Dictionary
User-submitted slang thesaurus and dictionary. Alphabetized and includes synonyms and antonyms of listed terms. Users can give each word a rank.
http://www.slangasaur.us

Slang-Dictionary.org
A searchable archive of colloquial English used in American society, the UK in the 18th and 19th centuries, Australia, hip-hop culture, modern-day London, and the American underworld.
http://slang-dictionary.org/

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

Southern Slang Dictionary
Colloquial American words and expressions originating specifically from Arkansas. Notes part of speech and includes example sentences. Sorted alphabetically.
http://littlerock.about.com/cs/southernlife/a/aasouthslang.htm