Did you ever dream of being able to generate astonishing 3D worlds? Have you ever desired to simulate phisical objects, collisions, and particles in space? Well, it is time to start, and the cost of commercial quality software is will not stop you. Blender is a marvellous open source 3D content creation suite that you can have free of cost at Blender web site.
Don’t be scared by the interface, which takes some time to master; after a couple of days you will see how productive this kind of GUI can be. There is a big number of free video tutorials and guides that will help you creating your masterpieces. Just start with the links provided below:

blender.org (Blender 3D project home page)
blenderart magazine
BlenderNewbies – a beginner’s resource for learning Blender 3D
BlenderWiki
Blender Tutorials at BlenderNation
Blender Tutorials Collection

If you are one of the lucky owners of one of the newest multi-core processors, you will be happy to know that multi-threaded rendering is supported, even though it must be explicitely enabled in the rendering parameters.
Blender comes with native support for an additional powerful external render: Yafray. Give it a try to see the raytraced image quality. The newest version of Blender has just been released. There are many noteworthy updates to this software; the most notable this time (at least for the average user) are Sculpt Mode and Multires Meshes. Have a look to this Blender page to learn about those features through videos.