Blockout is also very configurable. You can rotate blocks forward and backward along the x, y, and z axis (although all you really need is the x and y axis), and you can specify how many levels deep or high/wide when you play. A seperate high score is kept for each configuration for fairness. Support for multiple graphics modes is also well done, making Blockout playable on almost any machine.
My only complaint with Blockout is that there's no way to see empty spaces underneath the top layer. This is a hard problem to solve visually, so I'm not really blaming the designers (who did a great job). I guess I'm just lamenting my own memory's inability to keep track of 4 or 5 stacks of blocks. :-)" Tetris fans, eat your heart out with this great variant that is much better than Welltris, the "official" 3D Tetris.