STL is a file format native to the stereolithography CAD software
created by 3D Systems. This file format is supported by many other
software packages; it is widely used for rapid prototyping and
computer-aided manufacturing. STL files describe only the surface
geometry of a three dimensional object without any representation of
color, texture or other common CAD model attributes. (from wikipedia)

Stl files may be read by netgen and tetgen mesh generators.