3D slider puzzle for children, but surprisingly tricky even for adults. Helps learn 3D problem solving skills. Similar in concept to the 2D "fifteen" slider puzzle. Written in C++, this code compiles and runs on both OS-X and Linux. Now, improved function on laptops; runs on OS-X Retina displays too. Too easy, you say? Try Rufas' Cube, my 3x3x3 big brother.
There is also an Ada version available, as well as a flattened 2D version called Rufas7.
Mac OS-X or Linux with graphics card that supports OpenGL v 3.3