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.
You can find some snapshots in the Gallery.
NETGEN 4.9.12 is available from sourceforge: http://sourceforge.net/projects/netgen-mesher
beta downloads for windows can be found here netgen_download.