Wiktionary:Other dictionaries on the Web

"Proper" dictionaries

 * Cambridge Dictionary Online
 * Dictionary.com
 * Fine Dictionary
 * The Free Dictionary
 * HyperDictionary - includes Webster's Dictionary, 1913
 * LexiCool
 * Longman Dictionary of Contemporary English
 * Merriam-Webster Online - American English as main entries, also includes British English as 'See-also' entries. Dictionary and thesaurus.
 * Memidex - WordNet-based dictionary/thesaurus with definition, audio, and etymology references
 * OneLook Dictionary Search - searches many others - including Wiktionary, great for American and British English, checking spellings, wild cards (e.g. searching for words containing definitions and crossword-type stuff like searching for words spelled C_M_N) and definitions. Dictionary and thesaurus.
 * Oxford Dictionaries
 * WordNet - a lexical database for English
 * Wordnik
 * WordReference
 * World Wide Words
 * YourDictionary

General Public License dictionaries

 * GNU version of The Collaborative International Dictionary of English (GCIDE)

Public domain dictionaries

 * Hypertext Webster Gateway at BennetYee.org
 * Jargon File Resources

Slang

 * Texting Abbreviations List. Textspeak dictionary.
 * British Internet Slang Dictionary by Ted Duckworth
 * NoSlang - Internet Slang Dictionary & Translator
 * SlangLang - Online dictionary for slang words
 * Urban Dictionary - user-generated content, with an enormous database of genuine slang but also made-up-and-used-by-one-person crap.

Jargon

 * FOLDOC - Free On-Line Dictionary of Computing
 * NetLingo - The Internet Dictionary of Computer and Internet Terms

Translations to/from English

 * AllWords
 * The Compleat Lexical Tutor - free tools
 * Ethnologue: Languages of the World
 * ForeignWord
 * FreeTranslation
 * Glosbe: Bilingual dictionaries hub, over 100 languages
 * Linguee - search 100 million translations (German)
 * Translation.net - free instant translation from Systran
 * Larousse
 * dict.cc

Chinese

 * MDBG English to Chinese dictionary - interface to, a Chinese-English electronic dictionary with Mandarin pinyin
 * CC-Canto - Cantonese dictionary based on CEDICT
 * MOEdict - interface to several online dictionaries: Mandarin, Min Nan, Hakka dictionaries from the Ministry of Education (Taiwan), a cross-strait dictionary from the General Association of Chinese Culture. Translations from the CC-CEDict (Chinese–English), CFDict (Chinese–French) and HanDeDict (Chinese-German) are also included in entries.

Czech
Appendix:Czech bibliography

Danish

 * Den Danske Ordbog - current language; maintained by Dansk Sprog- og Litteraturselskab
 * Ordbog over det danske Sprog - covers the period 1700-1950; also published by DSL
 * Nye Ord i Dansk - a dictionary of recent additions to Danish; also maintained by DSN

Esperanto

 * Reta Vortaro - possibly licensed under GNU GPL V2 (click "libera" at the bottom of the page)

Faroese

 * Orðalistar - an array of topical word lists, some Danish-Faroese, some English-Faroese, some monolingual; published by Føroyska Málráðið, the Faroese Language Council
 * Føroysk orðabók - published by Orðabókagrunnurin, the Dictionary Association

French

 * Centre National de Ressources Textuelles et Lexicales
 * Larousse

German

 * Digitales Wörterbuch der Deutschen Sprache
 * Duden - grammatical information, synonyms, pronunciation and more
 * Wörterbuchnetz

Greek

 * η Πύλη - Greek language portal

Greenlandic

 * Katersat - Greenlandic/Danish/English. Some (most?) derived words have their composition given, and most have plural and relative forms and pronunciation. Affixes are listed independently, but there seem to be some lacking.
 * DAKA - published by Ilinniusiorfik (Greenlandic/Danish). According to this, it is a combination of earlier works Oqaatsit and Ordbogen.
 * Greenlandic English Dictionary - published by Oqaasileriffik, the Greenland Language Secretariat; downloadable spreadsheet (Greenlandic to English/Danish); includes both new and old orthography; still in progress.

Japanese

 * Jisho.org - online interface to several electronic Japanese–English dictionaries like EDICT, JMnedict, etc.
 * kotobank.jp - free access to the monolingual Daijirin, Daijisen, an encyclopedia, and a Japanese-English dictionary.
 * weblio.jp - ditto, with dictionaries of many specific domains.
 * dictionary.goo.ne.jp
 * eow.alc.co.jp - a Japanese–English dictionary

Scottish Gaelic

 * An Etymological Dictionary of the Gaelic Language

Spanish

 * Diccionario de la lengua española © 2005 Espasa Calpe
 * Real Academia Española - has two Spanish monolingual dictionaries: "Diccionario de la lengua española" and "Diccionario panhispánico de dudas"

Turkish

 * A Public Domain English-Turkish dictionary
 * A Public Domain Turkish synonym dictionary

Welsh

 * A pocket dictionary has about 25k one-line Welsh entries with English definitions, e.g. "Ystyriaethu, v. to use consideration or reflection" Proofreading is still in progress.