Gmsh (A three-dimensional finite element mesh generator with built-in
pre- and post-processing facilities)

Gmsh is an open source 3D finite element mesh generator with a
built-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, in ASCII text files using Gmsh's own
scripting language (.geo files), or using the C++, C, Python or Julia
Application Programming Interface (API).

See this general presentation for a high-level overview of Gmsh
and recent developments, the screencasts for a quick tour of Gmsh's
graphical user interface, and the reference manual for a more thorough
overview of Gmsh's capabilities, some frequently asked questions and
the documentation of the C++, C, Python and Julia API.

The source code repository contains many examples written using both
the built-in script language (see e.g. the tutorials and demos) and
the API.