Designing in 3D is exciting. If you are lucky enough to own a 3D printer (or have access to one), then I am sure you want to take advantage of it. However, your CAD skills may not be good enough to test out the printer. We have compiled a resource for FREE 3D printer models that can be used for 3D printing. This should help you get started!
This is by far the best resource. My Mini Factory is the world's largest library of 3D models for printing. All of the designs are user generated. You are free to use them as you wish to test out your 3D printer or even enhance them for projects.
My Mini Factory is great because their internal team reviews each design to make sure it is printable. Try them out!
Thingiverse is managed by the creators of MakerBot. There is a plethora of designs on their website for you to use. You can even share your own designs to help out others.
3D Warehouse is part of Sketchup. It houses several 3D designs that you can use.