The primary focus of Meep is simulating the interaction of light with Photonic Crystals (periodic materials that control light based on both their shape and composition).
You may have heard of optical metamaterials (aka "invisibility cloaks"). They are sometimes made with photonic crystals, but photonic crystals aren't always considered metamaterials. Butterfly wing coloring is a good example.
This is a cool example of solving a problem by developing a general purpose tool, then programming the tool to solve the specialized problem. It's cool for a couple of reasons. First, many aspects of the tool's behavior can be confirmed at the general purpose level, i.e., does it behave according to the expected laws of electromagnetism. Second, it provides the entire community with something useful.