It's free, it's fantastic, and it's runs on any laptop/desktop computer, it's SketchUp Make:
You can access it directly from the Google Grid, and sign in with Gmail.
Useful Keyboard Shortcuts:
Double tap to select a shape (face and edges)
Triple tap to select the whole 3D shape (all faces and edges)
M to move (objects or vertices)
Here's a ready made lesson a few from Helen Gamble: