3doku is a 3D version of sudoku (or, if you are a mathematician, a 2D version of sudoku wrapped around a cube). Each of the six faces of the cube is a 4×4 grid, which must contain the numbers 1–16. The other groups are bands going around the cube: pick any square, and move either vertically or horizontally, folding around the edges of the cube as you go. Once you reach the square you started from, you should have covered 16 squares — which should of course contain the numbers 1–16.
3Doku is also available as an Android app.