During university I and some other students created some very interesting games/mods.
Weatherman is a fun little Mod for Doom 3 that allowed the player to change the weather around him.
Color Sucker lets the player indirectly manipulate a balls projectory to solve puzzles and give the world its color back.
Also, I did make two big modifications for Doom 3 while the biggest one took around five years to complete.
Desolated: The Crying Fate was my second mod to be released and it is a huge story driven shooter-adventure with many different environments, game-modes, difficulties, secrets etc.
Hell Island for was my very fist release. It did use the main mechanics of Doom 3 while adding almost completely new environments. Gameplay top, graphics flop. Mostly.