That's right! You can download the entire Doom soundtrack RIGHT NOW for free via the Videogame Soundtracks download page. This OST package includes both the original files that launched on the first run of the game, as well as the remade files released as part of The Ultimate Doom. The tiles on the home page have also undergone an update that looks cleaner and far more arranged.
As for upcoming projects, you can't listen to the music of Doom and then just skip out on what comes next! So naturally, Doom II: Hell on Earth will be the next in line for an OST download.
What happens after Doom II you ask? Well... I'll just leave this here.