NETGEN is an automatic 3d tetrahedral mesh generator.
It accepts input from constructive solid geometry (CSG)
or boundary representation (BRep) from STL file format.
The connection to a geometry kernel allows the handling
of IGES and STEP files. NETGEN contains modules for mesh
optimization and hierarchical mesh refinement. Netgen is
open source based on the LGPL license.
It is available for Unix/Linux and Windows.