As at November 2007, there are 19,600,000 online dictionaries found when I just did a google.com search.

WOW!

Pretty amazing stuff!  Here are some of the best free online dictionaries, which you might find useful:

• The Macquarie Dictionary
  

  The well-known Macquarie Dictionary calls itself "the record of Australian English". First published in 1981.

• Merriam-Webster's Collegiate Dictionary, 10th Edition
  

  This is the one I used constantly!  Provides free dictionary and thesaurus look-up including an online audible pronunciation database.

• Online Dictionary Database
  

  This online dictionary query service searches multiple online dictionary databases and returns results instantly.

• Newbury House Online Dictionary
  

  Based on the best selling Newbury House Dictionary of American English, this online dictionary  contains over 40,000 entries .

• yourDictionary.com Portal
  

  This portal says it is the most comprehensive and authoritative portal for language and language-related products and services on the Web. Has more than 1,800 dictionaries for over 250 languages.

• One-Look Dictionary-Search
  

  This one is fantastic;  called the "Faster Finder", it claims to search 700+ online dictionaries simultaneously, and has a total word count of over 4 million.

• Xrefer Search Engine
  Xrefer's free site contains encyclopedias, dictionaries, thesauri [that's multiple thesaurus] and books of quotations from the world's leading publishers. An extraordinary source - one stop shop - providing all of these  cross-referenced, in one place.