PovRay

PovRay -- the "Persistence of Vision" ray tracer -- is an application that you pass a scene description file to (a text file describing, in geometric terms, some scene you'd like rendered). As output, it produces a picture of the scene you described. It does this by simulating the scene you told it about; it traces straight lines all throughout the scene, starting at the "eye position" (or "camera location") you specified, and bouncing around the scene.

If you're interested in free 3D graphics software on GNU/Linux, you should have a look at povray.