Many people here are mentioning the SNES, but actually, outside of classic SNES games like Star Fox, Stunt Race FX, and Yoshi's Island using co-processor chips,

Many people here are mentioning the SNES, but actually, outside of classic SNES games like Star Fox, Stunt Race FX, and Yoshi's Island using co-processor chips, SEGA also produced what they called the SVP chip for Virtua Racing on the Genesis. Then we got the SEGA 32x - which was literally dumping a giant chain of extra chips onto the Genesis in order to do things like primitive 3D and scaling. Beyond the SNES as well, the NES also had a habit of packing in small bits of extra hardware on their carts.