Blockout

Blockout is one of the best Tetris sequel that is not an official Tetris release. It's basically Tetris in 3D. Instead of looking at the well from the side, you look down into it, and rotate and place all of the pieces in 3D. It's a bit tricky to get used to, but many consider this game one of the definitive (albeit unofficial) iterations of the original Tetris formula. Welltris is the official 3D version and honestly I'm not sure how they differ, but Blockout seems to be the better known of the two.