T3D is a powerfull mesh generator capable to discretize complex 3D domains into triangular and tetrahedral meshes of high quality.
T3D2VTK is extensible and customizable utility written in Python to convert T3D output files into VTK files, which can be visualized e.g. in ParaView.
Example: Cube with spherical inclusions
T3D2VTK is a free software distributed under GNU LGPL license.
See documentation and/or examples for more information.