Gmsh is a free 3D finite element grid generator with a build-in CAD engine and
post-processor. Its design goal is to provide a fast, light and user-friendly
meshing tool with parametric input and advanced visualization capabilities. Gmsh
is built around four modules: geometry, mesh, solver and post-processing. The
specification of any input to these modules is done either interactively using
the graphical user interface or in ASCII text files using Gmsh's own scripting
language.

See also <https://www.geuz.org/gmsh/>
and <https://gitlab.onelab.info/gmsh/gmsh>.