Nethack is an open source, freeware role playing game known as a 'roguelike', being a direct descendant of a game known as 'Rogue'. The game is incredibly compact, being only slightly larger than what you could fit on a floppy disk, yet contains a near infinite wealth of gameplay.
You play an adventurer descending into a dungeon full of various types of monsters, trying to locate an amulet at the very bottom, and escape with your life, yadda yadda yadda. Much like so many other CRPGs. Where it differs is that every level in every game is randomly generated, with no two levels alike; every time you play the game, it's a different experience. Secondly, when you die, you die. No reverting to backup save games here. This extends the playability enormously, and makes you play a whole lot more cautiously.
I highly recommend this game. I've been playing it for at least six years. I've completed it at least once with every character class. I still play it now. In fact, I'm going to go play it.
Did I mention it's free?