I do some concept illustration, which includes 3D modeling at times. Just thought I would mention a program I just downloaded and started using.
Art of Illusion
It's surprisingly mature and full featured for a program I had not heard of before. And although it is built with Java Swing, which requires installing the JRE, updating to version 1.6 of the JRE gave me a significant performance boost.
You can do a little online test of which version of Java you might have installed
here.
In any event, if you haven't ever gotten into 3D modeling Art of Illusion isn't too difficult. A nice quickstart tutorial in the help section gives you some step-by-step instructions. You'll have your first render done in the time it takes to figure out where all the controls are on some packages. Detailed tutorials give you most of the details of features and 3D concepts.
The procedural editor is what I'm figuring out now. You can also download a variety of plugins for specialized tasks, like building plants.
So far it seems pretty powerful, especially for the price -- Zero. It's a nice project which deserves more support and attention than it is getting.