Nethack

This is seriously the best adventure game ever conceived. Download it for free here.

It's amazing what sorts of things you can do. The game functions much like any other RPG, except you only control a single character (the little @ on the screen). Battle and traps and other things run on a dice-rolling system for probability. There's an extensive stat system and inventory. The game is also randomized, so every time you play the game, the dungeons and enemies will be different. You also don't know what some of your items are. For instance, you might pick up a scroll entitled LEP GEX VEN ZEA. It won't say anything more than that. It could do pretty much anything, from generating a hundred monsters that attack you, to teleporting you somewhere random, to curing disease, to polymorphing an object or genociding an entire race of monsters. Each new game you play, those nonsensical scroll titles will be randomized too. The LEP GEX VEN ZEA scroll might teleport you in one game and kill you the next. Now of course, there are scrolls of identify, and other ways to tell what a scroll actually does (potions and spellbooks are randomized as well). Some character classes can also identify certain objects. Oh yeah, and there's food in the game. If you don't eat when you get hungry, you'll die. This forces you to press onward in search of food and gold.

Speaking of pressing onward, the game has upwards of 50 dungeon levels (I've never made it far enough to know the exact number). The object is to reach the bottom and acquire the amulet of Yendor, and then return to the top (ascend). This is insanely difficult. When I say this is difficult, I mean you will die. A lot.

If you needed another reason to play this game, it has Grues in it. Yeah, that's right.