Dwarf Fortress is a single-player fantasy game. You can control a dwarven outpost or an adventurer in a randomly generated, persistent world. The world is randomly generated with distinct civilizations spanning over 1000 years of detailed history, dozens of towns, hundreds of caves and regions with various wildlife. Command your dwarves as they search for wealth in the mountain. Play an adventurer and explore, quest for glory or seek vengeance. Extended ASCII character set rendered in 16 colors (including black) as well as 8 background colors (including black). Although Dwarf Fortress is still in a work in progress, many features have already been implemented. Please post all support issues in the forums for the game.
Most complex simulation, I've played so far. It's so complex it out performs modern CPUs! This is the one and only down side I've experienced, but if it gets nasty fewer ticks per second are absolutely ok.
One tip for players: expect the unexpected and have a lot of FUN. ;)
If you can get passed the odd ASCII graphics you'll discover one the deepest strategy games you'll ever come across.
A word of advice, go read a quickstart guide. It'll save you a lot of frustration. Installing a graphics mod probably wouldn't be a bad idea either.