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.
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.
Museum and Centers Listings from the Highland Park Elementary School, Austin TX, web site. Excellent links to major museums and centers.
"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.!)
PeoplePlay UK. A highly interactive site maintained by the Theatre Museum, National Museum of the Performing Arts in Covent Garden, UK. 1500 digitized photos of theatrical artifacts, plus lots of accompanying information.
The Smithsonian
[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]
Theatre Museum, the National Museum of the Performing Arts, a branch of the Victoria and Albert Museum (V&A), London.
Museums of Taiwan: Theatre Museums.
World Wide Arts Resources -- Art Museums. Comprehensive listing of museums, many with clothing and cultural artifacts.