NetHack is a single player dungeon exploration game for most platforms! Windows/Mac
What is NetHack?
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.
For information on running the program see the README that comes with each copy.
Still Playable on a Mac Pro with OS X 10.5.6! http://www.nethack.org/