SketchUp is a powerful tool set for creating incredible 3D models and graphics. It has a free and a Pro version. There is a very large subculture of 3D artists on the Internet that use a variety of 3D software packages; hardly any of them have a free version or are so easy to learn., few of them free. It works on PCs, Macs, and Unix.
I've worked with a lot of different 3D modeling programs and SketchUp is one of the most accessible and easiest to learn. Besides being free, it comes with hundreds of free video tutorials, many of them on UTube. There is also the SketchUcation forum and Magazine, and plugins, lots of plugins (little programs you can plug into the larger program to add additional tools).
I'm assembling some examples... soon, soon.
Parents...turn your son's and daughters on to this program. If they enjoy it, it will only benefit them, even if they never work in 3D later, it will stimulate their creativity, improve their computer skills, and it's fun.
You can get it at http://sketchup.google.com/intl/en/index.html
SketchUcation, the world forum for the 3D modeling software SketchUp. Over 500,000 members and a great starting point for learning about the software. Check out the WIP (work in progress) forum for outstanding work.