The Lewis Walpole Library has digitized 10,000 images from its superb collection of eighteenth- and nineteenth-century satirical prints -- not the only collection of its kind on the Internet, but certainly one of the largest and best. Search under "Gillray", "Rowlandson" or "Cruikshank" and browse a selection of images from the golden age of English caricature. Everyone will have their own favourites, but here are a few of mine: Rowlandson's Author and Bookseller, Cruikshank's The Headache and Gillray's Advantages of Wearing Muslin Dresses.
The Corpus Vitrearum Medii Aevi now has a digital archive containing 10,000 images of medieval stained glass from English churches and cathedrals: a wonderful resource for anyone interested in medieval art. These stunning images of the windows at Fairford, in Gloucestershire, are just a tiny fraction of the extraordinary riches available on the site.