A mod that uses Freedoom graphics, sounds, and sprites so they can be played in the official Doom IWADs. The mod is playable on a variety of supported source ports, including GZDoom, Zandronum, Chocolate Doom and derivatives, Doom Retro, and more!
The Ultimate Doom's E4M6 played with Freedoom in Doom 0.12.1. The skybox is quite beautiful, isn't it? Played through GZDoom 4.4.2.