Angband
Angband is a single player dungeon game with (originally) ASCII graphics.
But you can run across several clones with some other graphics like zangband.
The game is loosely based on the world of J.R.R Tolkien. Angband itself is an enhanced clone of moria. What makes it so fascinating is it's complex world where you seldom find yourself in the same situation twice. The dungeons are auto generated and contributes to a non-repetitive environment.
For more information see
The official angband page
or why not try the Tolkienpages at Lysator
I've seen from my logs that lots of people come here after searching for zangband using one of the search engines available. I'll be kind enough to show you a *much* better Zangband page
If you are a fan of Angband or games like that, this article could be quite amusing.