Nethack is one of those games that transcends time. It was originally written in the 80's and has been in more or less continuous development since then by a godlike group known as the DevTeam.
As you may have guessed from its age, Nethack is a text-based game (not like Zork; you have an overhead view of a dungeon and monsters, items, etc are represented by letters and symbols). Now, some of you may be wondering why I consider its graphics excellent despite this. This is because it does not need better graphics; they would, in fact, just get in the way. After a few minutes playing you can glance at the screen and see exactly what is happening without having to think about it. In the advanced stages of addiction you genuinely fear certain letters and will swear profusely if you see a D (dragon).
There are no sounds. I personally recomend puting a cd in the drive and listening to it (something atmospheric if possible, or rock; rock is very suited to the feelings evoked).
The game is difficult. So difficult. This is a game where completing it will be the pinnacle of many months of playing. This is a game which only has a save function to let you leave the computer for a bit (no reloading if you die...). This is a game which has you hack your way down through hundreds of levels of dungeon until you reach the bottom, at which point you have to hack your way back up again, pestered the whole way by an unkillable foe (the Wizard of Yendor) who will steal the big treasure from you every chance he gets. And, when you reach the top you will have to fight your way through the four hardest levels yet. And then ... then you get your reward; you ascend and become one of the elite, one of the people who have finished Nethack, who have ascended. What then? Then you go back and try again with a different class (there are 13, or so) or race or playing with some self imposed restriction (no weapons for example). And, above all, you will enjoy this.
The designers have thought of everything. You can kill a cockatrice (chicken/lizard/monster that turns things to stone), pick it up (assuming you have gloves) and use its corpse as a weapon, turning everything you hit to stone. You can train your pet dog/cat/horse to steal things from shops. You can do so many cool things that I am not going to even attempt to list them here. If you can think of something cool to do, chances are the designers have put it in there ("They thought of everything" is a common quote).
Above and beyond all this, you will have fun. This is the game that you will come back to again and again, long past the time you have forgotten all others. This is the game that Diablo failed to copy well enough. This is a game you will find yourself playing at 4am on a Monday morning before work, after starting play on Friday evening.
You will find yourself coming back to Nethack again and again. This I promise. You will play until you are so tired you can't sleep and then you will play some more. You won't care that you haven't moved in 16 hours.
I could say more, but I can't really describe Nethack. You have to experience it to understand.