Doom SysAdmin Tool
One popular modification of the Doom source code turned Doom into a system administration tool. Dennis Chao developed the first version of this and published a paper on the subject. David Koppenhofer later release psDoom, an enhanced version on the same theme.
The modification creates a Doom monster representing each process on a Linux system. Wounding a monster decreases the priority of the corresponding process, and killing it similarly ends the process. In psdoom, monsters are resurrected if the user does not have permission to kill the process.