Tetris. I play it occasionally to pass the time whenever the forums die or are devoid of anything interesting:
https://tetris.com/play-tetris
https://www.tetrisfriends.com/games/Sprint/game.php
https://www.tetrisfriends.com/games/Marathon/game.php
https://tetris.com/play-tetris
https://www.tetrisfriends.com/games/Sprint/game.php
https://www.tetrisfriends.com/games/Marathon/game.php