American Theatre Wing: Contains career guides and videos on working in the theatre.
Internet Broadway Database: the official database for Broadway theatre information. IBDB provides information on productions from the beginnings of New York theatre to the present.
Theatre Central: A fairly comprehensive site, linking to sites on many different aspects of theatre.
The WWW Virtual Library: Theatre and Drama: Contains links to plays and theatre books available in print, and also links to plays and monologues available on the web.