VTK

VTK (Visualization Toolkit) is open-source free software for 3D image processing and visualization. It supports various popular file formats, as well as having its own native format, which exists in both a "legacy" ASCII-based form, and a newer XML-based one.

Format documentation

 * VTK 4.2 file formats

Sample files

 * Sample VTK files at fsu.edu

Other links

 * Official site
 * Writing VTK files using Python
 * Some discussion of VTK file formats