Recs.
Updated
Open ended city-building simulator. Minecraft meets The Sims, with a difficulty level that can only be expressed through swear words.
Slaves to Armok: God of Blood Chapter II: Dwarf Fortress is a part roguelike, part city building game. It generates a high fantasy world using fractals and mathematical techniques so that each world is completely different. Like most roguelikes, Dwarf Fortress is text-based but don't think that means it's light on computer resources. DF is the most complicated and in depth game ever. Tarn and Zach Adams have worked on the game since 2002 and plan to work on it for another 20 years before it comes out of alpha.
SpecsUpdate
Pros
Pro "Starter Pack" offers a streamlined introduction
The DF Starter Pack, formerly Lazy Newb Pack, is a collection of community-created enhancements to the interface and visual presentation of Dwarf Fortress. Find it here.
Pro Sim-like "Fortress" and roguelike "Adventurer" modes
Dwarf Fortress offers two primary ways to play. In Fortress Mode, the player oversees a small group of dwarves who embark to build a new settlement. Individual dwarves cannot be controlled directly; instead, the player supplies objectives and constraints and the A.I. decides exactly what is done by whom. Adventurer Mode presents a more classic roguelike experience, wherein the player controls a single character and can explore the entire world (including previous player-made fortresses).
Two secondary modes are also provided: Legends Mode, which allows you to browse through all of the notable people, places, objects and events in a particular world's history; and the Object Testing Arena, where AI combat scenarios can be played out with any arbitrary combination of creatures and weaponry.
Pro Base game is free (supported by donations)
From the Bay 12 Games donations page:
Bay 12 is dedicated to providing original games free of charge, but that doesn't mean it is free for us to make them. We've been online since December 2000, and you can help Bay 12 Games continue to thrive. If you've enjoyed any of our games, and you are able to give, we accept financial support.
Pro Renowned for depth of simulation and emergent gameplay
The ultimate goal of Dwarf Fortress is no less than a total fantasy world simulator. Its simulated systems include geology, metallurgy, fluid pressure, temperature, combustion and phase transitions (melting, evaporation, etc.), shear and tensile strength, ballistics and gravity, anatomy and tissue damage, growth and maturation of animals and plants, emotional states, life goals, and alcohol dependence.
Cons
Con Controls are complex and often counterintuitive
New players are advised to read the Quickstart Guide.
Dwarf Fortress takes a lot of patience to learn. Even something as basic and early as world creation might take a guide to understand the first time around.
It doesn't really matter how experienced of a gamer you are; if you try to play this game without a guide, you're going to have a hard time.