Freedoom is a project to create a complete Doom II-compatible IWAD file which
is Free Software.

The IWAD file is the file used by Doom which contains all the game data
(graphics, sound effects, music, etc.). While the Doom source code is Free,
you currently still need one of the proprietary IWAD files from id in order to
play Doom. Freedoom aims to create a Free alternative. Combined with the
GPL-licensed Doom source code this will result in a complete Free Doom-based
game.

WWW: http://freedoom.sf.net/