For a full round-up, try NASA's 3D Reconstruction page:
        http://biocomp.arc.nasa.gov/3dreconstruction
and check the Medical Image Volume Visualization Software FAQ
from:
        [log in to unmask]
which is posted on several newsgroup such as:
        sci.techniques.mag-resonance
        sci.image.processing
(if you cannot find it, I have a copy available)
 
A favorite is VolPack from Phil Lacroute at Stanford.
It is solid, it runs fast on anything, and it is _free_.
It consists of a library, an interface to Tcl/Tk, two
examples of Tcl/Tk user interfaces and documentation.
 
One can easily customize one's entire user interface, extend
its functionality, even integrate it with one's own software,
image processing routines, statistical analysis, data base
and what not.
Quality of renderings is excellent and includes depth-cue,
multiple lights and materials and other features not found
in expensive commercial packages.
 
It installs easily on any workstation, and does not need any
special graphics hardware. The source code is provided. One
needs a compiler, Tcl and Tk, all of which being available
on the net.
 
There is a paper in the ACM Siggraph 1994 Conference Proceedings.
One can download the paper, and the software from:
        http://www-graphics.stanford.edu/
        ftp://graphics.stanford.edu/pub/volpack
 
 
  --Jean-Pierre Hebert