Welltris was the first Tetris sequel. It retains that game's falling-block puzzle gameplay but extends the pit into three dimensions while the blocks remain two-dimensional, with the board viewed from above.
As blocks descend into the well, they can be rotated or moved left or right along the walls, from one wall to another if desired. Once a block reaches the floor, it will slide as far as possible until stopped by an edge or another piece. Whenever the player completes a solid horizontal or vertical line, it disappears and the remaining squares slide to fill the open space.
If a falling block comes to rest with any part of itself still on a wall, that wall is temporarily frozen; no blocks can be moved onto it during this time. Freezing all four walls ends the game.
Source: Wikipedia, "[en.wikipedia.org/wiki/Welltris Welltris]", available under the CC-BY-SA License.