Wolfenstein 3D played a large part of driving my interest in computers and learning how they really work under the hood when I was young. Plus the release of MapEdit really made you feel powerful and a young kid getting involved in computers.

Doom is the point where I became interested in programming and it got me looking to how you actually made these games. id Software eventually open sourcing Wolf3D really helped spur that on. On top of that, Carmacks talk and his always interesting .plan file (a blog before blogs) made him seem very accessible and really opened up the world of game programming. I still enjoy going back and reading parts of the Wolf 3D or Doom code to see how they handled certain problems/limitations.

I'd agree with the other poster that 'Masters of Doom' is a great read and worth it to grab if you haven't read it already.