Blender is one of my favorite open source software projects. It is a full featured 3D content creation suite licensed under the GPL. My friends who work in the 3D and motion graphics industries tell me that the Blender feature set is currently not on par with the high end professional tools such as 3Ds Max or Maya. However, it does boast some impressive abilities and is available, for free, for your platform of choice.

To see what it's capable of, I suggest you check out the site for Big Buck Bunny, which is an animated short movie created entirely with Blender. For my money, it looks pretty darn good for something created with completely free tools.