Slic3r
Description
Slic3r is a free, open source program that was created for 3D animation projects, but is also extremely useful as 3d design software. It generally involves beginning with basic shapes (rather than sketches) and manipulating them into the preferred model, and includes a wide range of animation options.
Availability
Blender is open source software, anyone can download and install it for free on their personal computer.
https://www.blender.org/download/
Getting Started
Tutorials are available at http://www.blender.org/support/tutorials/.