Doodle in 3D. It's a simple 3d line drawing app based on mr doobs's funky harmony sketch thingy: http://mrdoob.com/projects/harmony
Once you're happy with your doodle you can save a screenshot or even the 3D drawing as an .obj file to mess around with in 3D programs, or 3D print maybe?
Just a wee toy to scribble, doodle, sketch. create intricate 3 dimensional objects and watch them rotate.
My kids keep stealing my phone to play with this, the toddler especially seems hypnotized by his spinning art :)