simpleFDTD is an open source embeddable 2-D finite-difference time-domain electromagnetic simulation It is designed to be a simple tool for getting quick answers when you're interested in knowing at what wavelength a plasmonic particle scatters as a function of size, material, and dielectric environment. It runs in a browser by compiling from C to WebAssembly to allow (semi) native performance.
Project link: https://simplefdtd.waldocorp.com