ASCII Rules OK
Only two games have been on the hard drive of every computer I have owned since I first encountered them: Civilization, and NetHack.
Technically, NetHack is a "Rogue-like game," Rogue being the first game of this type, originally implemented for academic minicomputers before there was such a thing as a home computer. Rogue-likes are D&D-ish dungeon crawls, with randomly generated dungeon levels, and wide variety in terms of monsters, weapons, armor, spells, and magic items. Rogue wasn't a bad game--but there's nothing like NetHack.
It's "Net" not in the sense that it's multiplayer, but in the sense that it's developed and maintained by an open-source development team collaborating over the Internet. And indeed, the NetHack dev team was iteratively improving the game long before rms coined the term "free software" or esr came up with "open source." (esr was a NetHack dev team member for a while, btw).
Playing NetHack reminds you of how bogus the modern game industry is--because a pure ASCII title delivers vastly more gameplay, and more fun, than 99% of the crap you'll find on the shelves at GameStop. Gameplay over glitz.
Playing NetHack can be frustrating; although there are occasional hand-crafted levels, mostly are randomly generated, and you can easily be screwed by bad luck. But it's quick to play, and deep, and will keep you occupied for... well, years, potentially. If hack-and-slash dungeon crawling sounds like fun, you're an idiot if you don't download this game. It's free, after all.
Hints
When your character dies, NetHack erases its last save file. So when you save, navigate to the directory where you saved it, copy it someplace else, and when you die, recopy to the Nethack directory and reload. This is "cheating" in some abstract sense, but actually, NetHack notices that you've done this (with occasional gentle chiding), but doesn't stop you.
If you get deep into the game, you will absolutely want the NetHack spoilers. We won't link from here, but do a Google search.
And... if you really want to beat the game, and know C, consider reading the source.