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.

For more information, see http://freedoom.sf.net/

