Welltris, which is the first Tetris sequel, is originally designed by Alexey Pajitnov (the creator of Tetris) with Andrei Sgenov.
It has the same gameplay concept as Tetris; simply all the player has to do is to rotate and maneuver the falling pieces into a proper position in order to form complete layers in the rectangular playground. But Welltris changes this rectangular arena into a four sided well with grid-like walls and bottom where the falling pieces can be moved from wall to wall within the well. Form the pieces into solid rows either horizontally or vertically on the bottom of the well.
Billed itself as Tetris in three dimensions.