Cambridge International Dictionaries Online. A collection of Cambridge dictionaries of English, they include Phrasal Verbs, Idioms, etc...

http://dictionary.cambridge.org/

Dictionary.com. Online searchable English dictionary, including links to other English and non-English online dictionaries and a glossary of English jargon terms.

The Exploding Dictionary. A sort of 'meta' search form including various publicly-available English dictionaries such as Websters, the Hackers Dictionary, and the Jargon File

Hypertext Webster Interface An online version of the Webster English Dictionary.

Oxford Reference: English Dictionaries and Thesauruses. Online, searchable versions of Oxford English dictionaries and thesauri, giving full definitions and etymologies.

 http://www.oxfordreference.com

WordNet - a Lexical Database for English. A searchable online English dictionary organised semantically rather than alphabetically, with full hypertext-annotated definitions, and including slang terms. You can also download your own copy from the site.

Yourdictionary.com. Anything having to do with words and dictionaries, from specialised dictionaries to glossaries of all types.

Travlang - English-Spanish Online Dictionary http://dictionaries.travlang.com/EnglishSpanish/

Travlang - Spanish-English Online Dictionary http://dictionaries.travlang.com/SpanishEnglish/

The Jargon Dictionary. A dictionary of English jargon and slang terms, with particular reference to hacker slang.

Webopedia.com . A searchable dictionary specialised in Computing. Recommended.

FOLDOC. Free on-line Dictionary of Computing. An easy-to-use searchable dictionary, type a word and you will have access to definitions and real examples.