Okay. I think this is the first time I've made a topic specifically looking for a certain sort of game. Or, rather, recommendations for a very specific kind of video games. To sum up what I'm looking for, see this elegant and finely crafted link:
http://tvtropes.org/pmwiki/pmwiki.php/Main/MarathonLevel.
I love games with replay value. I also like games that have enjoyable core concepts. Unfortunately, not all enjoyable games have a lot of replay value, and that's where this comes in: a dungeon, or cave, or maybe a temple, or WHAT HAVE YOU in-game that is a massive timesink in the sense that it has a truly gargantuan amount of floors/levels compared to other areas in the game.
The only other real requirement I have is that the game is not an MMO of any sort. I know WoW and such have several variations of No Social Life: The Dungeon, but I don't enjoy doing that while inside an MMO.
To better refine my preferences (though not at all requirements):
Better: Each floor is at least somewhat different/varied. (The marathon levels that tend to crop up as bonus content in certain Final Fantasy games, for example)
Worse: All floors have similar layout, enemy type, etc. (Disgaea's item world comes to mind, even though the world depends on the item)
Better: The dungeon/level/whatever is 100+ floors long. (I love the idea of an insurmountable dungeon)
Worse: The dungeon/level/whatever is less than 50 floors long. (A lot of "bonus dungeons" in RPGs)
Better: If the floors are procedurally generated, they're at least interesting (think Unangband's dungeon generation, only with the "interesting" amp'd up several levels, as implausible as that is)
Worse: If the floors are procedurally generated, they're more of the same, just with increasing difficulty (think a lot of early roguelikes)
Better: The game is not a roguelike. (I already know most roguelikes feature this, but with undesirable results due to the example in "worse" just above)
I think that's about it, really. I wonder if anyone will actually read this. Oh, and I've skimmed through the tvtropes article -- I've played most of the ones that sound interesting enough (usually with disappointing results)