I'm TAing an introductory class, and of course Miller indices and unit cells versus primitive cells and all that crystallographic stuff is a big first week. It's tough for some students to wrap their heads around all the 3-D symmetries, and *really* tough for me to draw all those structures on the board. I'd like to make a hw assignment where they build some basic structures, twirl them around a bit, and label some planes and directions using a computer. My department is broke, so the cheaper the better. If someone can suggest a visualization program or method that even sophomore undergrads can handle, that would be AWESOME. If there's a tutorial involved, that would be incredible.