Simple Media has a free vst version of this instruments for windows (for Cubase, Ableton, Fruity Loops), that is called Spook Keys (a name that is an homage to the Star Trek series). The plugin offers control either using a MIDI keyboard or using the mouse to drag the glowing spot across the 'chart' for more authentic 'Theremin' style play. This is the best Theremin VST emulation, and it’s free! Drag the light point on the amplitude / frequency axes: instant Theremin action! Here come the flying saucers. You can also play MIDI notes with your keyboard. A theremin is a musical instrument controlled by a performer with simple hand motions, and it is capable of generating eerie sounds that are used in concert music, movie soundtracks and more.

Free Windows Music Software: Spook Keys is a free theremin VST for Windows, designed to re-create the typical sound of a theremin. It supports control either using a MIDI keyboard or using the mouse to drag a glowing spot across the ‘chart’ for a “virtual theremin” playing style.

The Theremin !
One of the very first electronic instrument, invented by the Russian physicist and cellist Leon Theremin in 1919.

Everybody knows this instrument that you can play without physical contact. The Theremin produces a tone that changes in pitch and dynamic level when the player moves his hands near its antenna and loop.
It’s often associated with 50’s sci-fi movies, furious aliens and creepy/spooky creatures and was also used by many artists, just to name a few :
Beach boys – Good Vibrations !
Portishead – Mysterons
Goldfrapp – Yes Sir
Cinematic vst plugins free ableton 10. + Radiohead, Led Zep, Massive Attack, Pink Floyd, Julien Dragoul and many more !

If you search the perfect VST for this landing spaceship sound here’s a selection of free VSTi that tries to emulate the good old Theremin :

1. Spook Keys by Simple Media

Spook Keys VST

Download : http://www.simple-media.co.uk/vsti.htm

This is the best Theremin VST emulation, and it’s free !
Drag the light point on the amplitude / frequency axes : instant Theremin action ! Here come the flying saucers. You can also play MIDI notes with your keyboard.
You have 4 waveforms to choose from so there is more than the Theremin sound, check out the presets !
Delay is simple but effective.
Depth and Rate parameters are for the vibrato.

Alien invasion, they are coming
https://blog.wavosaur.com/wp-content/uploads/2014/06/spook-keyz.mp3

2. BTS Super Theremin

Best free vst plugin overdrive. Download it => here

This synth can do a decent Theremin emulation, add some reverb and/or delay after and you’ve got a nice clean gliding sinus Thereminesque sound. The perfect VST for science fiction movies!

BTS Theremin VST + cathedral reverb for more romantic SCI FI sound
https://blog.wavosaur.com/wp-content/uploads/2014/06/BTS-theremin.mp3

3. 666 VST by Eloxoga

the number of the beast

666
Click on the button, and drag the skull : a demonic Theremin from hell will scream.
Perfect for horror ambience or invoking the devil.

4. Mysteron by FXPansion

Download : https://web.archive.org/web/20050623003606/http://www.fxpansion.com/skunk/

Drag the little cross or use you MIDI keyboard.
This one is very simple, you may need to add a delay / reverb effect after.

5. Bluenoise Theremin

Bluenoise Theremin

Download for free here : http://www.bluenoiseplugins.com/plugins/free-plugins/

This one is funny, with the hand you can drag.
The range for the vibrato depth is not well calibrated, only the 3 first positions are really usable for Theremin emulation.
The signal is a sinus when you turn the “age” parameter to the maximum
Note that you have to send MIDI message.

Spooky Creepy Scary Funky Science Fiction SFX
https://blog.wavosaur.com/wp-content/uploads/2014/06/bluenoise-theremin.mp3

6. Super Spook Keys

Free download : http://www.simple-media.co.uk/vsti.htm

A slightly different version of Spook Keys.
A great VST for classic sci fi sound FX.

Free Theremin Vst

7. Enigmatron VST by FXPansion

Enigmatron VST

Free Theremin Vst

Direct DL : espace Cubase

This is a customized version of the Mysteron Theremin VST.

8. TerVoxSyn VST by Chris Roberson

Free Theremin Vst

Freeware download : http://robersonaudio.tk/tervoxsyn/

This free VST is worth mentionning too. It has a nice “voicy” sound.
Set the timbre to zero for a more “Theremin” sound