Nethack

Nethack is a very very old, all text PC game that has more depth, creativity, and challenge than virtually anything ever created since.

Official description: NetHack is a single player dungeon exploration game that runs on a wide variety of computer systems, with a variety of graphical and text interfaces all using the same game engine. Unlike many other Dungeons & Dragons-inspired games, the emphasis in NetHack is on discovering the detail of the dungeon and not simply killing everything in sight – in fact, killing everything in sight is a good way to die quickly. Each game presents a different landscape – the random number generator provides an essentially unlimited number of variations of the dungeon and its denizens to be discovered by the player in one of a number of characters: you can pick your race, your role, and your gender.