Q4tro is - in the first place - a high quality, polished Blockout 3D Tetris clone with a professional style documentation an website.
But furthermore it is an experiment in 3d interactions and 3D interfaces to 3d puzzles. it is not meant to be a serious blockout game. it is rather exploring new ways to interact with 3d worlds.
especially interesting is the “x-ray” mode (by pressing Ctrl), where blocks turn transparent, such that you can see where you made mistakes during stacking and left holes.
very well done, especially the high quality documentation is something outstanding. better than in most commercial games.