I wanted to do something interactive for Maker Faire last year. I had been drawing my PenFlakes, and thought it would be cool if people could design their own and print them out. So I created FlakePad, a javascript/HTML5 web app that enforces the basic symmetry of a snowflake, and provides a hexagonal grid to work off of.
Aside from being a great way to get my hands dirty with HTML5, the most interesting part of the app was creating the hexagonal grid. I wound up learning about and utilizing Isometric Cubic Coordinates. These coordinates provide an amazingly simple way to label hexagons on a grid, as well as a relatively simple transformation to and from standard Cartesian Coordinates. The basic trick is to recognize that a hexagonal grid, can be seen as a projection of a 3D arrangement of cubes centered on the plane x+y+z=0 (imagine Q*bert, the old NES game).