You might've seen the original Doom remade in DoomGL and other user-made engines that bring the Doom experience into a third dimension. Now you can experience the latest Doom title—Doom 3—in Doom, or something approximate to the experience.
With DeathFactoryX's Revenge of Evil TC mod for GZDoom, Doom 3's characters and textures have been converted into an engine that plays very much like the original Doom did.
The game is still in development, with the first completed map available for your viewing pleasure. DeathFactoryX plans to release the mod when he completes five maps.