Dictionaries

Know Play
It provides a neat front end to sources for answering quick reference enquiries. UK readers will find great benefit from using the general and reference sources.

xrefer
A single interface enables you to search a huge range of encyclopedias, dictionaries and quotations.

Bartleby.com
A library on a single site, Bartleby provides a dictionary, encyclopedia, thesaurus and much more.

Your Dictionary.com
The premiere website for dictionaries - a massive site of dictionaries of every speciality and language, you can even get translations of klingon. Scroll to get a full idea of scale of site then click until get to what you want - access to Thesauri, speciality dictionaries, grammars, etc.

OneLook Dictionaries
Nearly 4.2 million words in 746 online dictionaries.

Xlation
Xlation is a site primarily designed for translators. It features more than 1,600 dictionaries, which also serve as glossaries. Using the links on the Dictionaries page allows you to select both multilingual and single language dictionaries.

Cambridge dictionaries
Series of online dictionaries by Cambridge University Press including American English

Phrase & fable
On-line version of Brewer's Phrase and Fable

UK slang
An A - Z of UK slang terminology and English usage as it really is.