1- The Free Dictionary
This is my favourite web based dictionary. It supports several languages and it is also a thesaurus and offers acronyms, abbreviations, idioms, encyclopedia, search engine and literature reference library.

2- Dictionary.reference

The world’s most popular free online dictionary with definitions, spell check, word origins, example sentences, audio pronunciations, Word of the Day and more!

3- Merriam Webster

Free online dictionary, thesaurus, spanish-english and medical dictionaries, audio pronunciations, Word of the Day, word games, and many more high-quality.

4- Visual.Merriam
This is a dictionary that catches the eye and enriches the mind with more than 20.000 terms with contextual definitions, 6000 full-color images of a wide variety of objects.
5- Visuwords

Visuwords™ online graphical dictionary — Look up words to find their meanings and associations with other words and concepts. Produce diagrams reminiscent of a neural net. Learn how words associate.

6- Webopedia
An online computer dictionary and internet search engine for internet terms and technical support.

7- Wordia

Wordia is a free games-based learning platform – built on the foundations of a dictionary – that blends word-based learning games with interactive video vocabulary.
8- Your Dictionary

The easy to understand dictionary with example sentences, famous quotes and audio pronunciations. Includes: thesaurus, business dictionary, computer

9- Natural Science

A collection of quotations about science by scientists and other writers both ancient and modern.

10- Online Etymology Dictionary

History and evolution of more than 30000 words, including slang and technical terms

11- One Look

Type in a word and search many dictionaries at once. Also has a reverse dictionary and translations.

12- NetLingo

NetLingo has thousands of definitions that explain the online world of business, technology, and communication including the largest collection of Internet acronyms and text messaging shorthand

13- Music Dictionary

As its name indicates, this is a dictionary for musical terms
14- Medical Dictionary

The Medical Dictionary provides explanations of various medical terms and diseases.
15- Internet Picture Dictionary
Free, online multilingual picture dictionary with interactive activities for English, French, German, Spanish, Italian and ESL learners.