Planarity is a Flash puzzle game that is played by clicking and dragging the vertices of a 2D geometric shape to rearrange them such that no lines cross. It's just that simple. While the premise is straightforward, actually straightening things out is quite another story. In later levels the task becomes maddeningly difficult, and yet oh so sweet when you finally untangle the mess. It helps, of course, if you have even a mild case of OCD as I do.
Simple to understand, difficult to put down.