Blender is a free and open source 3D creation suite. It supports the entirety of the 3D pipeline -- modeling, rigging, animation, simulation, rendering, compositing and motion tracking, even video editing and game creation.
Alice is software that was created at Carnegie Mellon University, and is designed to teach junior high school and high school students computer programming in a 3d environment. Create animations, interactive games, videos, and share them.
LeoCAD is computer-aided design software for creating virtual LEGO models. This program has building blocks that allow students to create a virtual model. It's designed to allow new users to start creating new models without having to spend too much time learning the application. At the same time it has a rich feature set, enabling experienced users to build models using more advanced techniques.A bit tricky to start with, but worth the effort. Currently, about 10,000 different parts are available.