Doomworld has a re-released version of id Software's source code for Doom, which was previously released, now available as a GNU General Public License release, which alters the terms of the code's availability. It is now theoretically legal to sell programs based on the code, provided the new code remains freely-available under the GPL. Here's the accompanying quote from John Carmack:

Any DOOM based EXE would have to have the source available upon request. It doesn't necessarily have to be included, but you have to be able to get at it if you wish. This would prevent a gldoom episode.

The GLDoom episode to which he refers is the loss of the source code to one high-profile Doom source port to a hard drive crash.