Tip: Type "define:" in front of any word or phrase into the Google search engine, and it will respond with a list of definitions from a variety of online dictionaries and encyclopedias.
Online Etymology Dictionary
A free, well-referenced online dictionary of word origins.
Directory of online English dictionaries and thesauruses, translation dictionaries of 150 different languages, grammar help for 70 languages, and pages of linguistic fun.
American Sign Language Browser
From the Michigan State University Communication Technology Laboratory - this site features Quicktime video of thousands of signs.
An entertaining and enlightening peek into the lexicon of the online world.