123D Make is an app that lets you turn 3D models created in any tool but saved as STL or OBJ files into a real-world product made out of flat (2D) material that can be cut and assembled into a 3D model. While this may sound confusing, you'll instantly recognize some objects made in this way, most likely the 3D dinosaurs assembled out of 2D pieces of wood and sold in toy and museum shops worldwide. 123D Make is a 40MB download that is easy to install.
The 123D Make interface is a little overwhelming to start with, but you very quickly get used to it. There's a pane on the left with all the controls, most of which are drop-down lists. A big center pane shows a representation of the 3D model, and a pane on the right shows 2D cross-sections of the 3D model (which are what the app produces). You can adjust how the 3D model is sliced up and adjust several parameters, watching the model to see what it will look like when the 2D pieces are assembled. This might sound confusing but it really isn't, and when done right the 3D model from 2D pieces is attractive.
We made a few models from stencils drawn from paper (produced by 123D Make) on balsa wood, and they looked great. 123D Make worked flawlessly for us. While this type of app won't be for everyone, it is a fun app and a great way to work on stuff with others. If this sounds like a type of project you'd enjoy, 123D Make is for you.