English
Oxford English Dictionary (OED)
Oxford Reference
Over 400 dictionaries, language reference, and subject reference works published by Oxford University Press.
Arabic
Arabic Almanac
View, browse, and search Hans Wehr's Dictionary, Lane's Lexicon and Supplement, and Hava's Dictionary
Arabic Almanac iPhone App (Hans Wehr)
Digital version of the famous Hans Wehr dictionary, searchable in Arabic by root. Also available for Android.
A dictionary of modern written Arabic (Hans Wehr)
1961 edition available for browse and download via MENAdoc. English entries are searchable.
Chinese
Chinese Dictionaries and Reference Sources
Ancient Chinese to Modern Chinese 古代汉语字典
Modern Chinese to Modern Chinese 现代汉语大词典
Dutch
Woordenboek der Nederlandsche Taal
French
Le Grand Robert de la langue française
ARTFL: Dictionnaires d'autrefois
German
Deutsches Wörterbuch von Jacob Grimm und Wilhelm Grimm (DWB)
Greek
Online Liddell-Scott-Jones Greek-English Lexicon
Perseus Digital Library: Greek Dictionary
Hebrew
Modern Hebrew Dictionary (Lexilogos)
Portuguese
Michaelis Dicionário Português Online
Russian
Словари
Collection of dictionaries from the Institute of Linguistic Research
Фундаментальная электронная библиотека: Словари, энциклопедии
Collection of general and specialized dictionaries
Recommended Russian-English dictionaries:
Oxford Russian Dictionary
Katzner's English-Russian, Russian-English Dictionary
Reverso
Spanish
Real Academia Española: Diccionario de la lengua española and Diccionario panhispánico de dudas