Desolation is a Portal 2 mod, though that descriptor really sells it short. It’s closer to a full-blown, fan-made sequel to Valve’s series.
Set a few hundred years after Portal 2, and in a part of Aperture Labs that dates back to the 1980s, its star, Diana, “is fitted with Aperture’s BRACE cybernetic system, featuring a suite of robotic upgrades including an integrated Portal Device. When a relentless threat emerges, she must embark on a perilous journey that will see her uncover Aperture’s mysterious past… and maybe even face her own.”
While it’s far from done, it’s being worked on by some Portal 2 mod scene veterans, and is looking (and sounding, with its dynamic soundtrack) very professional. This seems a lot more likely to be completed and released in a reasonable timeframe than most other mods you see, especially when you consider they’ve been regularly and publicly updating everyone on their progress throughout Desolation’s three-year development.
You can check out some art and information at the projects mod page.