phanboy_iv writes: "Fans of both of the Raven classics Heretic and Hexen have been trying for almost a decade to convince Raven Software to release engine source code for both of these games under the GPL, much like the DOOM engine that both of them are based on. Well, they finally did it!. Sourceforge page here.
Both of these games have had the source available for a while, but under a restrictive license that hindered ports and modifications. Now, thanks to dedicated fans, that's no longer a problem."