With many windows source ports all over the internet, there still is limitless value to this classic game.
You play as a Marine in a Mars base, and your base is attacked by demons from Hell. The demons start killing everyone in the base, and you're the only one left. YOUR MISSION: kill everything in sight. But, you will need weapons in order to do that, right? Well, good news. The weapons in this game are excellent, and there's plenty of items to help you. There are LOADS of enemies to deal with in the game, and because of this, the game not only is loads of fun, but it makes you think hard to get past that cyberdemon or whatever.
If you have a DOS emulator, or better yet, a windows source port (as well as the original game), you should definately get Doom.