Here are a few links for free or cheap Macintosh 3D software.

Meshwork is a polygonal modeller aimed at small poly count models for games. It is a $30 shareware program.
http://codenautics.com/meshwork/index.html

Renderboy is a combined modeller and renderer. It is a $39 shareware program.
http://www.render-boy.com/

Celtic Assistant is a specialized program for generating Celtic-style knots. It can export a DXF of the final model. Celtic Assistant is free.
http://www.gigagraphica.com/ca/cainfo.html

Design Workshop Lite is a free version of Design Workshop, a modeller aimed at architectural illustration.
http://www.artifice.com/free/dw_lite.html

Patchdance is a 3D modeller using spline based patches. It is shareware, costing $25 for a basic version, or $75 for a more full featured option.
http://www.patchdance.com/

I second the recommendation above for POV-Ray. Though difficult to use, it produces fantastic output and is completely free. POV-Ray got me started in 3D.
http://www.povray.org/

I believe Pixels 3D 3.7 is released as freeware (the current commercial version of Pixels3D is 4.0).
http://www.pixels.net/home/index.html