The Getty Museum
Library of Congress Prints and Photographs (P&P) Reading Room Online Catalog. The collection is searchable, and much of the collection is in the public domain. Although the interface is Byzantine, it's worth the effort.
London Transport Museum. Excellent collection of railway film footage, posters and photographs. "Information Resources" provides links to essays and to related web sites.
The Louvre and the Web Museum
Museumland.com, "The World Wide Portal to Museums and Cultural Heritage", with more than 8000 links to museums in 118 nations.
"The Nine Planets" A media tour of the solar system. Intense graphics, good fun to tour. (Absolutely nothing to do with the theatre, but a very beautiful tour of our planets and their satellites.!)
V&A Theatre Museum Theatre Museum, National Museum of the Performing Arts in Covent Garden, UK. 1500 digitized photos of theatrical artifacts, plus lots of accompanying information. Very little of this material appears to have remained online following the reorganization of the web site.
[This is arguably one of the most maddeningly maintained sites on the WWW. URLs change without warning and seeming without reason, at an alarming rate. If you have difficulty locating Smithsonian sites, try backing through the URL until you find a redirect or index. JRW, November 2006. This is still the case in 2016!]
The National Museum of the Performing Arts, a branch of the Victoria and Albert Museum (V&A), London has been closed, its collections housed now in the Victoria and Albert Museum where it is but a shadow of this once wonderful treasury.
World Wide Arts Resources. Comprehensive listing of museums, many with clothing and cultural artifacts.