I first encountered one of these as a 100 in 1 or 150 in 1, sold at a local flee market. I remember it almost broke my brain seeing that thing. Not only had it more than one game on a single cartridge, it had so very many, but the cartridge was also a lot taller, maybe twice as tall as a regular one. And it was sold at less than the price of one official game.

I never got one, tho. My dad didn't allow me to buy one, saying these things were unethical and also saying these things were of an unknown quality and might break easily or may even break my Gameboy (not sure if he really was concerned about that, or said so to stop me nagging, as the ethics argument went straight over my 7 or 8yo head). But some of my friends eventually got one, so I regularly borrowed these things :P