I just encountered this great toy to challenge your three-dimensional geometry skills. The game in effect asks you to pretend you're creating an Escher-like puzzle, producing a an object that only looks right from three precise vantage points. Ever wanted to pretend to be a 3D rendering optimizer function? Now's your chance!
Warning: if you start it, you won't be able to stop until you complete all the puzzles. Which I did, although it took me an embarassingly long time.