Eviternity is a massive project put together by the Doom modding community consisting of six five-map episodes which use a new high-quality texture pack. The episodes each explore a different theme, with examples like "Icy castles", "Medieval", and "Industrial/Brutalism". Oh, and of course "Hell/Gore/Alien". There are a couple of secret maps as well, because what would Doom be without secrets?
Though it was initially released in December, Eviternity has just had a final revision, and you can read all about its changes here. Or you could just follow the instructions here to download all 49 or so megabytes of it.
Check out some screenshots below.