In 1999 id Software released the source code to their classic game Doom (under the GNU General Public License). The game as a whole did not get this treatment, however, which means that while the program code that powers Doom is free, the actual content of Doom - graphics, audio, levels, setting and characters - remains proprietary. The Freedoom project aims to fill this gap and create a complete free and libre first person shooter game.
By itself, Freedoom is just the content of a game and must be paired with a Doom-compatible game engine to be played: the download page lists some recommended programs.