NetHack

In NetHack, the most popular of Hacklike games, you must fight your way down through the Dungeons of Doom, and then the underworld (Gehennom), retrieve the Amulet of Yendor, fight your way back up to surface, and give the Amulet to your god. Notable features of NetHack include:

• The dungeon levels are persistant, so there is a finite supply of monsters and items, and if a particular level is difficult you must find a way to deal with it or circumvent it.
• Monsters can use the items that you use, so if a lowly kobold finds that Wand of Death before you do, you're in trouble.
• There are lots of little details and quirks in the game, which can interact in numerous ways for surprising and humorous results.
• There is no ready supply of Scrolls of Identification, and you start out not even knowing which scroll is the Scroll of Identification, so you have to develop tactics for identifying all of the items that you collect.
• The game will keep track for you various "conducts", so that if you're challenging yourself with various restrictions you can be sure that you haven't accidently violated any of them. Various challenges include pacifist (all monsers are killed by pets), illiterate (never reading scrolls or spellbooks), and even foodless (surviving without ever eating any food).