Other
National Portrait Gallery: Collections: Sitters a Z
This site from the official National Portrait Gallery of the United Kingdom is a resource to search the collection by typing in a subject's name or access the section on artists.
Art Cyclopedia
Artcyclopedia: Chronological Listing of Portrait Artists
ArtCyclopedia's complete list of portrait artists from the 15th century to today. There are links to the artists' biography and works.