There's an entire genre of music called "chiptunes", devoted to tweaking the sound of ancient videogame consoles, and a new Kickstarter project aims to put some of those controls in anyone's hands. The Gameboy Analog Filter (a name that wouldn't garner the Nintendo Seal of Approval) is a hardware low pass analog filter designed for the original Game Boy, and gives users some crazy tools to change how the music in their games sounds. It adds external controls for Cutoff, Resonance, Bypass, and Envelope Follower, and you can see their effects in the video below.
Unfortunately, this project isn't a cheap mod, and it's a pricy Kickstarter to back: $90 will get you a DIY kit to mount and install yourself, for $160 they'll install it on your own old Game Boy, and for $174 you get the Game Boy included. WIth that high of an opt-in price, it should be interesting to see if it can gain $2,200 of support in 16 days