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.

WWW: http://www.hpfem.jku.at/netgen/