Calculix

CalculiX is a free and open-source finite-element analysis application that uses an input format similar to Abaqus. It has an implicit and explicit solver (CCX) written by Guido Dhondt and a pre- and post-processor (CGX) written by Klaus Wittig. The original software was written for the Linux operating system. Convergent Mechanical has ported the application to the Windows operating system.
The pre-processor component of CalculiX can generate grid data for the computational fluid dynamics programs duns, ISAAC and OpenFOAM. It can also generate input data for the commercial FEM programs Nastran, Ansys and Abaqus. The pre-processor can also generate mesh data from STL files.
There was an active online community that provided support via a Yahoo! discussion group. Current discussion group is at Discourse. Convergent Mechanical also provides installation support for their extended version of CalculiX for Windows.
There is a friendly CalculiX Launcher with CCX wizard for both Windows and Linux.
Also possible is the Installation in Windows 10 Fall Creator (1709) with the new Linux Subsystem WSL.
The CalculiX solver is available on the Sun Grid.
A Python library, pycalculix, was written to automate the creation of CalculiX models in the Python programming language. The library provides Python access to building, loading, meshing, solving, and querying CalculiX results for 2D models. Pycalculix was written by Justin Black. Examples and tutorials are available on the pycalculix site.
FreeCAD has developed a FEM workbench which automates the creation of Calculix models.
There is a lot good examples of use of CalculiX by Prof. Martin Kraska, Brandenburg University of Applied Sciences.