Blender – 3D Modeling, Rendering, Animation, Video
Blender is the free open source 3D content creation suite, available for all major operating systems under the GNU General Public License. Its interface is tough to learn at first. But, after some hours with video tutorial instruction or books it becomes a very valuable asset to your personal production pipeline.

Google Sketchup – Simple, Powerful 3D Design
SketchUp is one of the simplest 3D programs to learn, and Google offers a free version! It is perfect for doing simple architectural renderings and layouts. Even the kids will love this one because of its ease of use. The free version can render architectural style sketches, and is very cool.

Bryce 5.5 – Free 3D Landscape Design
Bryce is a powerful piece of software that is capable of generating natural looking landscapes in 3d formats. Daz offers version 5.5 for download at Download.com. Upgrades to the latest version are usually inexpensive. Visit the Mac download page, or visit the Windows download page.