Hatris is basically Tetris with hats, and the reason why this is allowed is because the designer of Hatris is the same guy who made Tetris! Voila: no copyright issues with this clone. Ha, ha.
Alexey Pajitnov designed Hatris in 1990, just one year after the famous Game Boy version of Tetris came out. And Hatris was released into arcades first, with this PC Engine version (and NES and Game Boy versions) following shortly afterwards.
Like Tetris, Hatris features falling objects (in this case hats, of course) which you must arrange into neat piles. Stacking hats of different styles will raise the height of the piles, and if a pile reaches a set line at the top of the screen: it’s game over. Stacking the same hats in piles of four or more will make them disappear, giving you room to stack more hats, which is the aim: to keep stacking hats for as long as you are able to. And – like Tetris – Hatris gets more difficult as you progress, with hats falling faster the longer you play.
Hatris is one of those ideas that sounds terrible on paper, but works really well as a game. It’s different enough from Tetris to be interesting, and stacking hats has a weird compelling nature to it.
Hatris is what I would class as a “hidden gem” – if you’ve never played it, it’s well worth a look.
More: Hatris on Wikipedia