By this, I mean actual rip-offs, not mere clones (such as Centiped to Slither).
Donkey King was common, and there were many others, especially the Pac-Man rip-offs ("Puck-Man 3," "New-Puck 2").
And one rip-off arcade game, the one for Naughty Boy, was actually in the arcades before the legit one!
This was back in the 1980s. But, come to think of it, I didn't see that many in the 1990s and onward, except for a Street Fighter 2 rip-off that let you shoot diagonal fireballs. And I only saw that once.
So, to you more recent gamers, how often did YOU see rip-offs? How often do you see them now?