FEMSuite is an open source electromagnetism simulation tool, developed by Fady Abdelhamid Kamel.

It currently supports Yee's method for simulation, with experimental support for anisotropy.

Key features include support for parametric media, perfectly matched layers, and an extension (scripting) engine that allows solving optimization problems, such as determining an optimal brillouin zone to achieve a polarization-specific bandgap.

Source code
Source code documentation