ndc Plugs - Brush Strokes v1.0 (Pc/Free)

BrushStrokes is a FREE audiovisual instrument.

An audiovisual instrument with a drawing-based interface. Sound is generated in a manner similar to scanned synthesis, with two oscillators whose waveforms are obtained by scanning the x- and y-axis positions of the trail drawn by the user. The larger the gesture, the louder the sound, and vice-versa, when the mouse is not moving, no sound is generated.

From the homepage:
BrushStrokes. It's a simple idea (you play it by drawing), but actually quite powerful too, since gesture, sound and visuals are all very tightly linked.