Art's Theremin Page: Modern circuit designs for the theremin.
Jerry's Tube Theremins and Trautoniums: Builder of prototypes of different kinds of theremins and 2D21 Thyratron-tubed audio devices. Working on several versions of Trautoniums and Tannerins. Usually sells prototypes on Ebay. Uploading new images with progress.
Levnet -- Friends of the Theremin: The oldest theremin forum on the internet. Full archives and subscription form.
Moog Music: Owned and operated by Robert Moog, manufactures theremins and other high-quality electronic musical instruments.
The Musical Saw and Theremin Page: Information about theremins and musical saws. History, news, soundfiles, and upcoming performances.
Theremin Enthusiasts Club International: Information about the club and the instrument.
Theremin World: Theremin information including bands who use them, photos and sound, message boards, where to buy theremins, how to build theremins, and a growing registry of the original RCA models.
Theremin.info: Buy a theremin, music, books, CDs, and a large archive of related materials, including a number of sound files to download.
Thereminvox.com: Articles and interviews about the theremin and other electronic musical instruments, a theremin MP3 library, a theremin web directory and a shop.