What is "freedoom"

Freedoom is a free first-person shooter under continuous development. The project distributes three IWAD files: the single-player campaigns named Freedoom: Phase 1 and Phase 2, and FreeDM, which contains a collection of deathmatch levels.

The project presents itself 1 as complementary to the free source code of the Doom engine released by id Software in 1997 under the GPL license. It allows custom levels and other customizations designed for Doom or Doom II (e.g. PWAD files) to be used with Freedoom.