Yafaray is a free opensource ray tracing engine:

http://www.yafaray.org/

Some guy wrote simple 3D rendering PHP script. It is based on well known game called Wolfenstein 3D “raycaster” algorythm and is pretty simple. Funny is that you can combine that with AJAX to get some unseen results (let’s not think about server load and traffic yet).

Here a very cool example

Other links:

3d library
Image 3D live browser rendering
Dynamic canvas demo
Creating 3D with PHP

Blender is a free opensource 3D suite.
Blender was originally developed by Dutch company NeoGeo, then made available as a commercial product by subsidiary Not A Number. When NaN shut down, one of its founders set up the not-for-profit Blender Foundation and worked to raise funds to buy the rights to Blender. Consequently, Blender is now under the GNU General Public License which makes it open source and free for anyone to use.
In 2008 Blender and Big Buck Bunny have made the 12th place of the annual CG Retrospective!
Look at these movies to understand Blender potentiality.