Vimidi

That's right - you can use your mouse and keyboard as a MIDI controller. Vimidi application translates your actions on regular input devices to MIDI messages.

Your mouse X and Y coordinates are translated to MIDI messages, so if you move your mouse on the screen it will act as moving a fader on real MIDI controller.

So basically your screen becomes a XY screen controller. Mouse tracking is enabled by pressing and holding a hotkey.