DoomWorld: Doom Source Now GPLOct 03, 1999, 19:11 (6 Talkback[s])
(Other stories by Linguica)
"id Software's John Carmack has officially re-licensed the Doom source code to the GNU General Public License! For those not In The Know, the GPL license basically allows anyone to do whatever they want with the code, even modify and sell it, as long as the source remains freely and publicly available."
"This means that if a person wished, they could modify the Doom source and sell a game based off of it, providing that all the art, music, sounds, and levels were different (id still holds a copyright on everything included in the IWAD)."