Chocolate Doom is a port of ID Software's Doom, Doom II, and Final Doom to
modern operating systems that aims to be as compatible with the original DOS
executable as possible. This includes emulating all the different versions,
playing demos perfectly accurately, playing "vanilla-only" PWADS, and netcode
that works on newer operating systems while still behaving the same as
"vanilla" Doom. Additionally, Heretic, Hexen as well as Strife are also 
supported.

WWW: http://chocolate-doom.org