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.
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."
Bay Area English Regency Society — San Francisco bay area group that "celebrates the spirit of the early 19th century." Hosts balls throughout the year. Site has links to other Regency sites.
Before the Automobile — Blog by Finnish woman about 18th & 19th century clothing. Includes lots of photos of her beautiful creations.
Bibliothèque des Artes DécoratifsNew! — (In French) Large online collection of fashion plates from the late 18th to 19th centuries. To Search: 1) Click "Recherche simple"; 2) Type "mode" in "Termes de recherche"; 3) Select "Album Maciet" in "Document numérisés"; 4) Click "Rechercher"; 5) Wait until the results display in the left pane; 6) Check one or more of the results; 7) Click "Voir les notices"; 8) Under each selection in the right pane, click "Voir les vignettes Maciet" to see the fashion plates.
Regency Colors — A page with examples of colors, like Coquelicot and Jonquil.
Regency England — Articles on regency clothing, popular colors of the time, cosmetics and more
Regency Fashion Page, TheUpdated! — Update: The original site on locutus.ucr.edu is gone; the site on regencyfashion.org seems to be an attempt to replace it, but most of the links are broken. A few still work and can be worth the effort of the hunt (IMO). Great site!! Covers most aspects of fashions of the era. Sections on extant clothing, fashion plates, portraits and styles by year. (Site seems to be partially at "http://locutus.ucr.edu/~cathy/reg3.html" and partially at "http://regencyfashion.org/")
Regency GentlemenNew! — "Dressing for a Regency ball (and other events)", a blog from Sweden with a gentleman's perspective on Regency clothing and dancing.
Regency WorldUpdated! — Candice Hern's collection of regency items, including fashion plates, prints, silhouettes, quizzing glasses, reticules, buckles, etc.
Republic of Pemberley, The — Discussion groups and information pages about Jane Austen and her works. No real costume content, but lots of info about JA movies.