Although not as widely used as the theremin, the Novachord was another early electronic instrument often used to creepy effect in sci-fi/horror soundtracks. I first heard the Novachord on one of my favorite '50s exotica records, "Polynesian Percussion," by Lawrence Welk's bandleader George Cates. I figured it was just an electric organ, like the Voxx or Farfisa. It had a fascinatingly sleazy sound that I fell in love with, but rarely heard again. Now I know why. According to Synthmuseum.com, "The Hammond Novachord was first marketed in 1939, and was on the market until 1942. 1069 of them were created before World War II brought an end to production...they are quite rare. It is unknown how many still exist, and of those that do, few of them are operational due to the immense amount of tubes and capacitors, etc. they require. The device weighs five hundred pounds, and is as large as two spinet pianos..."
One brave soul who spent countless hours painstakingly restoring a Novachord was Phil Cirocco, whose "Novachord Restoration Project" page has extensive documentation of his labors as well as many fine mp3s, like this bit of soundtrack music from an episode of the old TV show "Outer Limits":
Harry Lubin: From "Demon With A Glass Hand"
Cirocco's conclusion after completing the project: the Novachord is, in fact, a synthesizer. Lacking the pitch-bending and abstract sound capabilities of Mr. Moog's creation 25 years (!) later, the Novachord thus was played like a conventional keyboard, albeit a somewhat odd-sounding one.
Cirocco's own solo Novachord music takes full advantage of the instrument's unique timbres - his short instrumentals have an Eno-esqe scope and feel that drop the listener into a pleasingly alien landscape.
Phil Cirocco: Improvisation #3
An album is in the works.