In 1998 the Doom source code was released. Since then, there have been a large number of Doom source ports created by programmers who have extended the code, adding new features and removing some of the limitations of DOS Doom (which is now often referred to as "Vanilla Doom"). These efforts should be commended, and many of the modern Doom source ports are truly impressive pieces of work.
Chocolate Doom, however, takes the opposite approach. It instead aims to accurately reproduce the experience of playing Vanilla Doom. It is a conservative, historically accurate Doom source port, which is compatible with the thousands of mods and levels that were made before the Doom source code was released. Rather than flashy new graphics, Chocolate Doom's main features are its accurate reproduction of the game as it was played in the 1990s.
The source code to Chocolate Doom is Free Software under the GNU General Public License, and is portable to a variety of different operating systems. It therefore preserves the gameplay of the original game for modern computers, many of which are now unable to run the original.
Chocolate Doom is written by Simon Howard, with extensive help from many other people who have provided patches, bug reports and suggestions.