The games Nintendo didn't want you to play: Tengen
Recently, I took a look at Nintendo’s MMC line of mappers, and some other boards. All boards for the NES’ western releases had to be manufactured by Nintendo, and so they generally met certain standards set by Nintendo. But these rules were enforced by technology, not by law. And the company that had previously killed the American game industry decided to break those rules. Madness? No. This… is Tengen.
Lots of custom cartridges here.
Some additional info: https://hackmii.com/2010/01/the-weird-and-wonderful-cic/