3D Slash, based in France, has just released a free version of their 3D modelling app (also called 3D Slash) to download on their website. You need to register but once you have you have access to this point-and-click software that will enable you to prototype just by clicking on blocks in the view area. It looks fairly low-res, but it’s a great starter app to get you used to the concepts involved in designing for 3D printing. Read more here or visit their website. See a video of it in action below: