http://www.bay12games.com/dwarves/
Well, the full (intentionally ridiculous, I think) title is:
Slaves to Armok: God of Blood
Chapter II: Dwarf Fortress
and I've got the feeling that this is one of those games that everyone but me already knows about (it having been out since 2006), but I just discovered it and am intrigued after having an hour or so to play it over the weekend.
Anyone who hasn't heard of it ought to read this, and keep going as long as they can:
http://afteractionreporter.com/2009/02/09/the-complete-and-utter-newby-tutorial-for-dwarf-fortress-part-1-wtf/