Dictionary.com is the largest and most trusted free dictionary on the web. Hosted by Ask.com, it contains a dictionary, thesaurus and even encyclopedia searching. Searches the Random House Unabridged Dictionary, American Heritage Dictionary of English Language, Merriam Webster's Revised Unabridged Dictionary and the American Heritage Dictionary of Idioms along with many more.
Etymology is the study of the history of words. This online resource contains explanations of what words meant in the past.