American Memory (Library of Congress) - Searchable collection of online historical images.
Costume Page - Costume and costuming-related links.
Costumer's Manifesto - Very comprehensive collection of costume links.
Fashion Era - Hundreds of pages of fashion history, costume, clothing and social culture.
Fashion Plate Collection - Digitized fashion from American and European journals.
History of Costume - Web version of the book Costumes Through the Ages.
Picture Collection Online (New York Public Library) - Historical images of clothing, dress and accessories.
Timeline Fashion - Timeline of the past century in fashion.