Dwarf Fortress

Dwarf Fortress is an
amazingly original game. It's basically a city builder
(ignoring Adventurer mode, as I do), but with the best procedurally
generated, umm, everything, of any game I've ever heard of. Yes,
this includes Spore (which isn't out here yet, but there's been much
written on its procedural generation, so I'm fairly confident here).
Yes, I'm serious. Every single dwarf (of which there can be
hundreds) and every animal and every monster is managed
individually, on the fly, transparently. For the dwarves at least,
the game rolls up a personality and uses that personality to
interact with everything around them, constantly. The terrain is
procedurally generated, and interacts in complicated (if not always
realistic) ways with everything else in the game.

In short, at any given time, any range of absolutely crazy stuff can
happen, giving a very, very massive "whole is more than the sum of
its parts" feel. The
Saga Of Boatmurdered gives a very good feel for the staggering
complexity of this game.

Unfortunately, absolutely everything about playing the game is
harder than it needs to be, and the interface is truly atrocious. I
don't mean that it's character based: I couldn't really care less
about that (although Dwarf
Fortress Graphics does make the game more palatable, even to a
long-time roguelike player
such as myself). I mean the user interface itself is horrifically
baroque, and everything you want to do is more complicated than it
needs to be. I've written about this in great detail at
The City Building Game I Want, but in short, a game should not be
utterly unplayable without
a vast,
massive wiki, and this one most certainly is.