Wiktionary:Other dictionaries on the Web

English 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

See also: Public domain sources

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.

MDBG English to Chinese dictionary - interface to CEDICT, 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.

Den Danske Ordbog - current language; maintained by w: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

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

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

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.

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

Real Academia Española - has two Spanish monolingual dictionaries: "Diccionario de la lengua española" and "Diccionario panhispánico de dudas"

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.