Tarn Adams just released new version of Dwarf Fortress.
For those of you unaware of this game, I offer the following introduction:
Take some dwarves.
Place them outside a mountain.
Survive.
It's fascinatingly complex yet incredibly difficult, and I heartily reccomend it if you have the time.
Here's a example of game depth, spread out over 40+ pages:
http://fromearth.net/LetsPlay/Boatmurdered/intro.html