MCX is a GPU-accelerated, general-purpose, physically-accurate and feature-rich 3-D light transport simulator. It is one of the fastest simulators because it can use tens of thousands of GPU threads to simulate photons in parallel.
CO2 is a real-world simulator development package that integrates currently
available technologies in order to provide an ever wanted environment for
those who want to build simulators/games. The package includes an IDE called
VisualGameBuilder.