Google SketchUp version 8 is now available, for free.
booklets from 3Dvinci which look to be a good buy - available for a reasonable cost each as a pdf download and with heaps of good tasks utilising SketchUp. Every student should have a play with this!
You can obtain some free instructions in YouTube, by looking for 'bonnie roskes' (author of 3Dvinci). For example, there are videos on drawing tesselations, an octahedron, an icosahedron (from three orthogonal golden rectangles) and creating the conic sections (very simply).