SpaceTheremin is a virtual mouse-controlled theremin. Unlike a standard analog theremin, which typically has a sinewave oscillator and some basic vibrato, SpaceTheremin lets you choose from six different oscillator types (sine, square, triangle, sinc, saw, and pulse). The same waveforms can also be used for vibrato and modulation. It uses PortAudio for sound generation and works on versions of Windows from XP through Windows 10.
Use of the vibrato can result in some incredibly freaky and bizarre sounds. Set the vibrato depth (in percent) and adjust the vibrato frequency via the slider to hear the effects. A vibrato depth of 50% and a triangle waveform gives an interesting sound, for example.
SpaceTheremin is 100% freeware and has been around for nearly a decade with tens of thousands of downloads. It is a fun toy/tool for creative noisemaking.