UK

Birmigham Slang.

Cockney Rhyming Slang.

Cockney Rhyming Slang.

Cockney Rhyming Slang Dictionary.

Glesga Glossary: slang from Glasgow.

London Slang: c’è anche il rhyming slang.

Slang Dictionary: English slang and colloquialisms used in the United Kingdom.

Slang Dictionary.

Victorian Slang Glossary: 19th century British slang – lower class and underworld.

USA

alphaDictionary.com: historical dictionary of American slang.

AmeriSpeak: expressions of our American ancestors.

Bay Area Hip Hop Dictionary.

Seattle Lexicon.

Silicon Valley Slang.

Slang City.

Slang Dictionary.

Slang Dictionary.

Slang Dictionary.

Slang Search: you can browse the dictionary by category or by alphabet.

SlangSite.com: with examples.

The Online Slang Dictionary.

Urban Dictionary.

Australia

Aussie Slang.

Aussie Slang.

Aussie Slang and Sayings.

Aussie Slang Dictionary.

Australian English Slang Dictionary.

Australian Slang.

Dagree’s Great Aussie Slang.

List of Australian Slang Words.

Ireland

Dublin Slang Dictionary and Phrase-book.

Irish Slang.

Irish Slang Dictionary.

Slang Throughout Ireland.

New Zealand

Kiwi Slang.

South Africa

RSA Slang.

Drugs

A Dictionary of Slang Drug Terms: with trade names, and pharmacological effects and uses (file pdf).

Drug Slang and Street Language.

Drug Slang & Terminology Vault.

Methamphetamine Slang Names.

Slang Drug Terms.

Street Drug Slang.

Street Terms: anche in versione pdf.

The Drunktionary: collection of slang terms for “drunk”.

GLBT & Sex

Dictionary of Queer Slang and Culture.

Gay Slang Dictionary.

Glossary of Gay Slang Terms.

Polari: British gay slang language, common in the 1960’s.

Queer Slang in the Gay 90’s.

Starma’s Adult Humor: sexual euphemisms.

Internet

Internet Slang.

Internet Slang Dictionary.

List of Internet Slang Phrases.

Military

Aviator Slang.

British Colonial Military Terms and Soldier Slang.

Glossary of Military Terms & Slang from the Vietnam War.

Great War Military Terms and Soldier Slang.

Naval Terminology, Jargon and Slang (qui c’è la parte 2).

U.S. Navy Slang.

Vietnam Veteran’s Terminology and Slang.

Various

60’s Slang.

Alternative English Dictionary.

Antarctic Slang.

Double-Tongued Dictionary: a lexicon of fringe English, focusing on slang, jargon and new words.

English Slang Dictionary.

Gangster Slang.

Hardboiled Slang.

Hard Boiled Slang Dictionary.

Jazz Slang.

Playground Slang.

Roadie Slang.

The Probert Encyclopedia Slang Dictionary: covers over 16,000 English language slang and dialect terms from around the world.

Abbreviations.com.

Acronym Finder.

Acronym Search.

Acronyms and Abbreviations.

Acronyms and Abbreviations.

The Internet Acronym Server.

Bulfinch’s Mythology: dictionary of phrase and fable.

Idioms and Phrases @ THe Free Dictionary.

Talk It Easy: il sito italiano degli idiomi inglesi.

The Idiom Connection: English idioms.

AllWords.com: English dictionary with multilingual search.

Cambridge Dictionary Online.

Dict.org.

Dictionary.com.

Dictionary @ infoplease.

Dizionario del Corriere della Sera: tratto dal Sansoni Inglese (EN<->IT).

Fact Monster.

Garzanti Linguistica: EN<->IT (la registrazione è gratuita).

get-definition.com.

Hoepli: EN<->IT (l’iscrizione è gratuita).

HyperDic: hyper-dictionary.

HyperDictionary.

Longman Dictionary of Contemporary English Online.

Merriam-Webster.

Neologisms.

OneLook: dictionary search.

OneLook Reverse Dictionary: lets you describe a concept and get back a list of words and phrases related to that concept.

Online Etymology Dictionary.

Oxford Paravia Concise: EN<->IT (registrandosi gratuitamente è possibile ottenere più funzioni).

RhymeZone: rhyming dictionary and thesaurus.

The American-British British-American Dictionary.

The American Heritage Dictionary of the English Language.

The Free Dictionary: online dictionary, encyclopedia and thesaurus.

Thesaurus.com.

Thesaurus @ infoplease.

Ultralingua: EN->DE-EO-ES-FR-IT-LA-PT.

WordNet: provides meaning, hyponyms, synonyms, hypernyms, derived forms…

Wordorigins.org: etymology.

Word Reference: EN->ES-FR-IT-PT.

Wordthrill.

WordWeb Online.

Your Dictionary.