Doom 3 has a huge, unofficial story expansion, and it's just been relaunched

Doom 3 is far from the best entry to the long-running FPS series, but a moody atmosphere, created by impressive lighting technology, still makes its Martian setting a compelling place to explore. It's also helped a group of modders called Team Future to use the game as the foundation for a sprawling expansion, Doom 3: Phobos, that adds a lengthy, unofficial story spin-off to the game well worth checking out. Now, as part of GOG's effort to streamline its users' ability to easily find and install mods, Phobos has been relaunched on the platform.