Abiti Antichi — In Italian. Lots of beautiful pictures of someone's collection of extant clothing from the 18th century through the 1930s. Easy to navigate, even if you can't read Italian. There is one page in English that explains the site.
Amica LibraryNew! — "Art Museum Images from Cartography Associates." Art image database with over 90,000 items from museums worldwide.
Artcyclopedia — Art can be searched by medium, location (i.e. museum), title, subject, and of course artist.
Australian Dress Register — "The Australian Dress Register is a collaborative, online project about dress with Australian provenance pre-1975. This includes men's, women's and children's clothing ranging from the special occasion to the everyday."
Bridgeman Art Library — Bridgeman has over 350,000 images online from multiple museums all over the world. "the Bridgeman Art Library works with museums, art galleries and artists to make the best art available for reproduction. The result is an outstanding archive of images drawn from collections throughout the world, all of which are available for licensing."
British Library: Online Gallery — The British Library has a large online selection of manuscripts and other documents. Includes a section of "virtual books", including several from the Middle Ages through Renaissance eras. (My favorite is Jane Austen's "History of Englad".).
Canadian Historical PortraitsNew! — Collection of portraits from Canadian history, from the 16th-20th centuries. The collection was created by Aegidius Fauteux, which was acquired by the Bibliothèque du Montréal.
Deliberately Concealed Garments ProjectNew! — The DCGP site features clothing found hidden in walls. From shoes to hats and everything in between, real extant clothing in various degrees of decomposition.
Enluminures (Illuminations)New! — (In French) This site has more than 120,000 images from over 5,000 medieval manuscripts and illuminations, kept in municipal libraries throughout France. Click "recherche guidée" from the home page to find manuscripts by author ("auteurs"), title ("titres"), type and context ("contextes"). You can also search by Library ("Accès par Bibliothèque"). Or, from the home page, click "recherche experte" to search using your own criteria.
Fan Museum, TheNew! — The Fan Museum in Greenwich, London. A small portion of their collection is available to view online.
Fashion Museum at Bath — If I ever get to England, this and the V&A are my two "must sees." Not a lot of images on the website, but definitely worth checking out.
Germanisches National Museum — Wonderful collection of medieval and renaissance art & textiles. Search & collections pages are only in German.
Harvard LibraryNew! — The Harvard Library website includes a visual search engine. From the main page, go to the "Search & Find" menu, and select "VIA". They have many interesting collections, including original costume design drawings.
McCord MuseumUpdated! — Located in Quebec, McCord Museum has lots of costumes and textiles, paintings and photographs covering Canadian 18th and 19th century history.
Metropolitan Museum of Art, The — The Met has a fabulous collection of clothing, The Costume Institute, as well as lots of beautiful paintings. Lots of images online — check out the Timeline of Art or just Search the Collection.
National Museum of DenmarkNew! — This site is in Danish, but with the help of a translation webiste, you can search for items like "kjole" (dress), "bukser" (pants), "kjol" (skirt), etc. Lots of lovely 18th century clothing in their collection.
Textile Museum of Canada — Museum has a large collection, from all over the world and spanning over 2000 years, most of which is available to view online.
Textile Museum, The — Part of George Washington University in Washington, DC, this museum was started from a personal collection and now contains textiles from all over the world and many different time periods.