The Longman Dictionary of Contemporary English is free online, and the dictionary is also available for purchase on CD. It attempts to keep up with current English usage as it changes with the times. A feature of this dictionary is dual definitions for words with different meanings in British or American English. You can use the search box to find all words in the LDOCE, have the words pronounced, and see an example sentence using the word.
The online version contains over 200,00 definitions. You can also look at the dictionary definition you looked at just previously by using your browser backspace button.
Longman is also affiliated with a website, Pearson Longman, with English Language teaching resources for students and teachers, as well as writing practice and free example exams in English proficiency
The LDOCE also offers a number of widgets which allow you to put the search box on your browser, Google homepage, or or your website.
The Free Dictionary is an online dictionary, Encyclopedia, and Thesaurus. It is published by Farlex. In English, it offers a Dictionary/Thesaurus, a Medical Dictionary, a Legal Dictionary, an Encyclopedia, and other free resources. Farlex also offers a subscription to the Hutchinson Encyclopedia. They provide phonetics for their definitions, and you can click an icon to hear the word pronounced. They also note when a word is used defined differently in American English than in British English.
If you like the features of the Free Dictionary, it offers you the option to set it as your home page, with a number of customizable features.
An entry in the Free Dictionary provides the etymology of a word, definitions from several sources; when I looked up 'boot', definitions were credited to The American Heritage Dictionary of the English Language, Fourth Edition copyright ©2000 by Houghton Mifflin Company. Updated in 2003, and Collins Essential English Dictionary, 2nd Edition 2006 © HarperCollins Publishers 2004, 2006, and the thesaurus section following the definition of the word came from WordNet3.0 Farlex clipart collection, © 2003-2008 Princeton University, Farlex, Inc., and Collins Essential Thesaurus, 2nd Edition 2006 © HarperCollins Publishers 2005, 2006. Below the thesaurus section was a section for translations, where you could select the language you wished to translate the word. This section was credited to Password English Learner’s Dictionary © 1986-2008 K Dictionaries Ltd and partners. All rights reserved.
Each entry in the Dictionary/Thesaurus also tells you in which other dictionaries on the the site the word is defined, such as the Medical dictionary, the Legal Dictionary,and Idioms. Sources for the Medical Dictionary came from Dorland's Medical Dictionary for Health Consumers. © 2007 by Saunders, an imprint of Elsevier, Inc. All rights reserved, the Saunders Comprehensive Veterinary Dictionary, 3 ed. © 2007 Elsevier, Inc. All rights reserved and McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc..