Earlier this year, Bethesda added free community-created add-ons (also known as WADs) to DOOM and DOOM II. It started off with the release of unofficial fifth DOOM episode SIGIL and now, even more, are on the way.
In an update on Bethesda.net, the development team announced it had released another free add-on, this time for Doom (1993). It's James Paddock's Deathless and is a 7.16 MB download. Here is the description, along with some screenshots:
James actually provided the original soundtrack for John Romero's SIGIL and whipped up Deathless in a single month. His own add-on includes 25 "fast, punchy levels" that offer a fair challenge and pure fun.