It might be stretching the definition a little, but this Doom II mod is one of the most technically impressive things I've seen in a long time.
For reference, this is what Doom II looked like upon release.
While this is what the upcoming Total Chaos - Overgrowth looks like.
Some of the more contemporary additions being made include 16x motion blur and bloom lighting, while smart use of a filter glosses over some of the areas the engine can't be dragged kicking and screaming into the modern era.
Note that this isn't a straight mod, though. The code from a 1994 PC game simply couldn't handle all that extra, new stuff. Instead, it's an all-new game being that'll run on GZDoom and Zandronum, two projects that take the original Doom source code and bolt on stuff like the ability to add things such as lighting effects.
Total Chaos - Overgrowth is intended as a no-guns game set in a post-apocalyptic future. Inspired by STALKER the idea is to explore a wasteland and "use their wit and the hazards they find in the environment to defend themselves".
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Total Chaos - Overgrowth [Site]