ImLib3D is an open source C++ library for 3D (volumetric) image processing. It
contains most basic image processing algorithms, and some more sophisticated
ones. It comes with an optional viewer that features multiplanar views,
animations, vector field views and 3D (OpenGL) multiplanar. All image
processing operators can be interactively called from the viewer as well as
from the UNIX command-line. ImLib3D's goal is to provide a standard and easy
to use platform for volumetric image processing research. Focus has been put
on simplicity for the end-user or developer.

WWW: http://imlib3d.sourceforge.net/