Scene Design
Jim Jenkins - (NY design resources)
Period Style for Designers  
Computer Scenographic Studio(Darwin R. Payne)
Medieval Drama Resources(Comprehensive! Plays, design, costumes...!)
Development of Scenic Spectacle - Dr. Frank Mohler
Joy of Visual Perception - A Web Book
AdFlip- Fabulous reproductions of period advertisements
Opera Philly - EXCELLENT renderings, drawings, etc
SLG Designs - LOTS of excellent musical designs!

Stagecraft
ESTA Home Page-TheatreSuppliers
Center for Safety In Arts
Tools for Stagecraft
Tech Theatre Humor
The Prompt Corner - Stage Manager site
Cobalt Studios , a professional painting studio that teaches scenic art!
STAGERAFT - Listserver Sign-up and information
Animated Knot Tying!
Glossary of Technical Theatre Terms

Lighting Design
Theatrical Lighting Resources on the Net
Lighting Archives Penn State University
Electronic Theatre Controls
Fireworks F/X, Inc.
Theatre Computer Visualization Center - Excellent Lighting source!

Costume Design
UGA Paris Music Hall Collection
History of Costumes - Braun & Schneider - c.1861-1880
Costumes of English Actors at the turn of the century
The Costume Site
MUSEUMS:
The National Gallery of Art in Washington, D.C.
The Metropolitan Museum's Costume Institute in New York City
The Museum of Costume in Bath, England
PERIOD BASED SITES:
The Elizabethan Costuming Page
Regency Fashion Plates, 1790-1825
The Victorian Library and numerous links
La Couturiere Parisienne Costume & Fashion , over 90 pages
Petticoats and Crinolines
Medieval and Renaissance Patterns
OTHER MUST VISIT COSTUME SITES:
The Costumer's Manifesto: The Best of the Best
The Costume Page: One of the Best
The Costume Ring
The Costume Site
Fabric Links
Young Designers Gallery: Must see and follow it to their library
AdFlip- Fabulous reproductions of period advertisements

Sound
Theatrical Sound Designers
Sound Design Listserver 

Acting
Acting Workshop Online

Directing
 

Playwrighting
New Dramatists Home Page
Public Domain Play Scripts
The Playwriting Seminars
Ontological-Hysteric Theatre (Free Scripts)

Dramaturg
What is a Dramaturg?

Theatre History/Literature
Theatre History on the WEB
Shakespeare Resources
Hypertext Shakespeare
The OXFORD Shakespeare
Mr. William Shakespeare and the Internet (Superb Site!)
The CLASSICS Archive at MIT
American Variety Stage(Library of Congress)
Medieval Drama Resources(Comprehensive! Plays, design, costumes...!)
New York Public Library for the Performing Arts
Theatre Research Directory (COMPLETE dramatists list)
Greek Stagecraft
Inter-play , an index to plays in collections, anthologies and periodicals - searchable.
Development of Scenic Spectacle - Dr. Frank Mohler
Links related to chapters in Brockett's History of the Theatre
1800's theatrical Ephemera
The Schubert Theatre Archives (EXCELENT design and architecture info too)
Theatre History LINKS - excellent

Magazines/Publications

Theatre Companies/Schools
YALE - School of Theatre
Outdoor Drama
Kids 4 Broadway - Children's Theatre

Professional Associations
USITT Pages
Southeastern Theatre Confrence
ATHE