Via Boing Boing:
Doom 1 has been ported to Doom 3 — you can play the original game on a terminal in its sequel.
The Terminal DOOM demo is a DOOM3 port – of sorts – of the Classic DOOM source as originally released in 1997. The playable demo is available for Windows and Linux, and supports all shareware and retail versions of DOOM. You will have to have the retail version of DOOM3 installed, and you will have to apply the version 1.3 patch to be able to run the Terminal DOOM demo. Once you applied the new patch, download the demo here from the mirror kindly provided by Ryan Gordon.