International Association of Libraries and Museums of the Performing Arts
National Association of Schools of Theater
Theatre Communications Group - Organization for non-profit theatre in America.
Theatre Library Association - Also includes a nice selection of links under "Related Theater Resources".
American Theater Web - links to regional theaters nationwide
American Variety Stage: Vaudeville and Popular Entertainment 1870-1920.
Backstage.com - look here for jobs!
Center for Research in Early English Drama (REED)
Didaskalia: Ancient Theater Today
Encyclopedia of Canadian Theatre on the WWW
Facets - Multimedia distributor.
History of Costume by Braun & Schneider
Internet Movie Database - Phenomenal database of just about everything you would ever need to know about any film ever produced.
Internet Broadway Database - Format based on the Internet Movie Database, details of Broadway productions.
Obie Awards - Off-Broadway awards from The Village Voice.