Getting classic shooter Doom running on things you didn't even know were computers (lamps, business desk phones, etc) is a genre unto itself. But now kgsws has outdone them all, by getting Doom running inside Doom itself. Jump 8 minutes in if you want a little technical context but not 8 minutes of technical context.
A note from the creator:
I have found a code execution exploit in the original DOS Doom 2 and ported a Chocolate Doom to it. And then Chocolate Heretic. Attention: This does only work on the original DOS Doom2 version, no GZDoom or other source ports. This is a good thing as you don't want code execution exploit on modern systems. People would abuse it to spread malicious code.
A suggested soundtrack: