Look at Blender
. This is a full featured, free in all senses, 3D rendering program. And from the examples I have seen, you can do amazing things with it. Otherwise, I seem to recall Google Sketchup
can do a good number of things, still no cost.
[Extra info] Sketchup is Mac and Windows only. Version 7 is free to download, the Pro version costs money.
Blender is Win, Mac, Linux, Irix and Solaris. May even be in your repositories.