Pete Edwards of Casper Electronics shares his method for modding the classic NES console to generate synth-synced visuals and more -
It is a very simple bend and is a lot of fun to play with. To bend this unit I simply added a patch bay to a handful of points on the video processing chips. The Display can be tweaked by either connecting points together or by feeding in external signals, like audio or voltages from my modular synthesizer. the video shown above is an example of how the visuals can be controlled using clock signals from my modular synth.
Suggested solder-points and more documentation can be found on the project page.
Related: Modified circuit bent NES