Theatrehistory.com, a wildly eclectic commercial site, with links to books, articles, sheet music, etc., from every century. Worth a visit in desperation, or for fun.
Theatre & Dance Research Guide, Yale University. Rich in links.
Theatron. This site, based in the UK, has an excellent link to 3D drawings of Florimène, as well as similar drawings of theatres in the classical world. To access these links, click "Gallery" on the homepage. It is also the link to major UK and European activity in the field of Virtual Reality and 3D architectural imaging. A very fine site.
Theatrum. This site has excellent links to theatres and performance activities throughout the world.
Theatre and Drama Links at Oxford Brookes University.
University of Maryland University Libraries: Theatre. "This is a guide to the important reference sources for research in the field of Theatre."
Vitruvio.ch Links to sites presenting general history as well as theatre historical sites -- e.g., the Teatro Farnese, in a 360 degree panoramic virtual tour. Excellent pictorial resources for virtual every century.
USITT's Wading On the Web (WOW)
Worthwhile Links, an eclectic collection of links related to the Arts, Children, General Humanities, Literature and Culture, Science and the Social Sciences.