To get it install Fractal Tune Smithy, Try the Mouse and PC Keyboard music too, and the other features...
This is the mouse theremin - as you move the mouse around in the window, the pitch and volume change. It's named after the theremin which is controlled by waving your hands in the air. You can actually get theremin controllers for a PC but as most don't have those, this lets you try out the same idea with a mouse, joystick, or from the keyboard.
There's a version of the mouse theremin with sliding tetrad chords (major and minor type chords going up / down the harmonic and sub-harmonic series). Here's a recording of it - your soundcard will need to be able to change its pitch bend range to hear this properly
You can use the joystick instead of the mouse if you have one.
You can also control it from your PC keyboard This makes it suitable for use by blind users for instance.
The way the keyboard interface works is that you hold down one of the eight numpad keys 7, 8, 9, 4, 6, 1, 2, 3, to change the direction, in combination with the Shift or Control keys for intermediate directions, and you use one of the ordinary number keys 1 to 0 to change the speed with which it moves, or the key to the left of the 1 to stop it.
Many options available for configuring it - this screen shot of the two configuration windows for it will give an idea. When you use the program you can press F1, or click on the ? icon to go to the section of the help that explains how to use the window
It comes as part of Fractal Tune Smithy and is a freeware feature of it.