MEEP a free finite-difference time-domain (FDTD) simulation software package

From the MEEP Website

"Meep (or MEEP) is a free finite-difference time-domain (FDTD) simulation software package developed at MIT to model electromagnetic systems, along with our MPB eigenmode package......"

For more information on MEEP,

1. MEEP Manual
2. MEEP Download
3. MEEP FAQ