WOLFSIM is a Finite-Difference Time-Domain electromagnetic simulator, designed to be easy to use but still very powerful, developed and maintained by researchers at North Carolina State University. It's features include:
-1D, 2D, and 3D structures that are periodic in 1 or 2 dimensions
-Materials that are anisotropic in permittivity and conductivity
-Obliquely incident sources
-Built-in vectorial (i.e. full polarization) near-to-far-field transformation
See these publications for full details on the algorithm:
http://www.ece.ncsu.edu/oleg/files-wiki/6/62/SPIE_12_Miskiewicz_wolfsim3D.pdf
http://www.ece.ncsu.edu/oleg/files-wiki/1/13/OptExpress07_OH.pdf
