Rez is a unique synthesizer program for the Nintendo Gameboy by nitro2k01. It crudely simulates a resonant filter in a way inspired by the resonance algorithm used by the Casio CZ series phase distortion synthesis. It can both produce sounds meant to emulate this effect, and sounds more reminiscent of a hard sync oscillator. It also produces a graphical pattern on the Gameboy screen.

Source via NESdev.

http://blog.gg8.se/wordpress/2012/12/11/rez-a-unique-gameboy-synth-rom/