They used to have these carts back in the day that had over 30 NES games in one cart...and this was back when I was in Greece in '92. They'd just work in a regular NES, but with a little fiddling around.
It's kinda like the ones they sell at the mall. It'll advertize like 300 games, but it's really the same 20 games or so, but with different settings (difficulty, extra lives, other cheats like that).
My cousin and I were walking through the mall a couple weeks ago and saw Contra...we stopped and played a 2-player game for a few minutes...I totally still rock that game.