Archiving Early America contains a variety of sources useful to the study of eighteenth-century America: maps, portraits, music, period newspapers, even the obituaries of prominent figures. A few books, such as David Ramsay's Life of Washington, are also available in their entirety. The site also hosts forums for the online discussion of historical issues.
The American Experience, a PBS series, includes a program on John and Abigail Adams. The companion site for this program includes excerpts from their correspondence and a synopsis of John Adams's evolution as a revolutionary.
The Mt. Vernon Estate and Garden website offers an extensive collection of photographs of the grounds and outbuildings, and a virtual tour of the mansion. The images may be of interest to the casual "virtual tourist," but they also offer students interested in material culture a fairly extensive introduction to the architecture, furniture, and styles of the period.
Alexander Hamilton and the Creation of the United States is an online exhibition hosted by the Gilder Lerhman Institute of American History. It includes portraits and a variety of documents, including the letter written by Hamilton to be delivered to his wife in the event of his death.
On a lighter note, check out this animated introduction to the XYZ Affair.