Triangle (It is not a freeware, until proved otherwise)

for generated Delaunay triangulations

Triangle generates exact Delaunay triangulations, constrained Delaunay triangulations, conforming Delaunay triangulations, Voronoi diagrams, and high-quality triangular meshes. The latter can be generated with no small or large angles, and are thus suitable for finite element analysis. Show Me graphically displays the contents of the geometric files used by Triangle. Show Me can also write images in PostScript form.