Open source Shakespeare, an excellent way to search the plays for individual words.
One Look, a dictionary search engine listing all the online dictionaries in which a word appears. Useful for comparing different definitions. One of their links is for Noah Webster's original 1828 dictionary, the first distinctly "American" dictionary. Check it out.
Dr. Samuel Johnson's 1755 dictionary.
American Verse Project, part of University of Michigan's vast array of online resources (Go Ohio State!). Its limitation is that it only cites American poems published prior to 1920 (still, very good for Whitman, Dickinson, Poe, John Greenleaf Whittier, other 19th century American poets.)
A slightly facetious, but perhaps useful site called word detective.
An online etymological dictionary, not a scholarly work, but a fascinating project being done by an ambitious amateur.
The Middle English Dictionary, another (gulp!) resource from University of Michigan.