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Merriam-Webster (Dictionary, Thesaurus, Spanish Dictionary, and Medical Dictionary)
Each retrieved word is accompanied by pronunciation, usage, grammatical function, a brief etymology, and a definition. The thesaurus can be queried for similar words, hypertext cross-references are available, misspellings return suggested spellings, and there are hypertext links to illustrations.
The Oxford English Dictionary
This site is the best resource for a complete history of a word and its usage in different ages. You can even receive emails with the word of the day.
This site is an e-text based on the 1911 edition.