Dwarf Fortress

Slaves to Armok: God of Blood, Chapter II: Dwarf Fortress, is a freeware computer game, more commonly known as simply 'Dwarf Fortress'.  It is a roguelike game, but features a common engine which runs over different modes of gameplay.  Gamplay varries from traditional dungeon crawls, to a sim city simulator.  The game is still in development, and some features are not as polished as they could be.  The scope of the game is very ambitious, with several more modes of gameplay planed before completion.  The game leverages it's common engine for what are in essance different games, and brings the game modes together, so that activities done in the dungeon crawl can effect the sim city mode, and vice verse.  The game is very large in scope, and attempts to use emergent properties from simple systems to create interesting simulations.