Tinkercad by Autodesk

Tinkercad is a free*, easy-to-learn online app anyone can use to create and print 3D models. No previous design experience required— intuitive, step-by-step lessons guide you through the design process from basic shapes to more complex creations.Tinkercad makes it easy to create anything you can imagine—from coffee cups to castles—in minutes, using the easy step-by-step instructions. Choose from more than six million designs in the Tinkercad Gallery.

Why we like it: I liked the designs available in the gallery. They made it really easy to pick something fun to print. Really good  step-by-step lessons on how to use each function to get a correct and wanted finished end result.