English language dictionaries

Bartleby.com
Cambridge dictionaries online
Dictionary.com
The Free Dictionary
Internet Dictionary Project
Merriam Webster collegiate dictionary
Webster dictionary
WordNet 3.0 vocabulary helper
YourDictionary.com

Multilingual dictionaries

Allwords.com
All Free Languages Project
ARTFL French-English dictionary
Ectaco online dictionaries English-Spanish language resources for library and information professionals
Ethnologue: languages of the world
FreeLang.net : free resources for languages
FreeTranslation.com
Le grand dictionnaire terminologique
I love languages (Human Languages Page)
IFLA multilingual dictionary of cataloging terms and definitions
LEO Deutsch-Englisches Wörterbuch
Majstro-Aplikajoj : a multilingual translation dictionary
Multilingual glossary for art librarians
TravLang translating dictionaries

Geographic information

Other reference sources

Subject/Discipline dictionaries

Accounting terminology guide
Artlex Art Dictionary
Biology Online dictionary
BusinessDictionary
Catholic encyclopedia
Canadian law dictionary
ChemiCool chemistry dictionary
Chemistry-Dictionary.com
Dictionary of botany
Dictionary of forestry
Dictionary of genetic terms
Dictionary of mining, mineral, and related terms
Dictionary of philosophical terms and names
Duhaime's law dictionary
Encyclopedia of psychology
Engineering dictionary
FAO fisheries glossary
Finance glossary
FishBase
Geology dictionary
Glossary of financial and business terms
Glossary of forestry terms in British Columbia
Illustrated glossary of geologic terms / Iowa State University
Jurist legal dictionaries
Law.com dictionary
Medical dictionary
Medical dictionary online
MediLexicon : medical dictionary, medical terminology
MedTerms
Nolo's plain-english law dictionary
ODLIS : Online dictionary of library and information science
Oilfield glossary
Paleontology glossary
PhysicalGeography.net glossary of terms
Stanford encyclopedia of philosophy
Stedman's online medical dictionary
VentureLine MBA glossary