NW Circles is a free MIDI generator that turns circular motion into evolving musical patterns. It's built around a system of rotating circles and trigger points that lets you create rhythms, melodies, and textures in a fun, visual way.
It lets you shape your sound with a range of little sliders for speed, probability, octave shifts and other note behaviors. Fine-tune how often notes play, how long they last, and how much variation is introduced.
It also features a bass layer that produces sustained notes based on your dot placements, with its own probability and timing controls.
There is no full version, Circles ships free, forever.
More information on the Gumroad page.