The Library of Congress catelog of early dance manuals - excellent resource for dance manuals through from Arbeau to contemporary works. Good source for 19th and 20th C manuals, like Dick's Quadrille Call Book.
The Library of Congress video collection - An incredible resource, miraculously paid for by our tax dollars.
Joy and Jealousy - a web book on early dances. Another terrific resource.
Nicolas Graner's excellent work on Arbeau's Orchesography. In French, so you can also work on your language lesson at the same time you reconstruct your dances.
Susan and Ken Reed's excellent site on clothing, including this page of hat types, Renaissance dance and Duello.
Andy Jewell's Vintage Dancing Bibliography - a listing of dance texts.