Dwarf Fortress

Dwarf Fortress

Dwarf Fortress is a single-player roguelike fantasy game. You can control a dwarven outpost or an adventurer in a randomly generated, persistent world.

Rule a fortress, fend off invasions of goblins, dragons, and undead elephants. Trade with humans, dwarves and elves.
Build traps, tombs, towers, and all kinds of dwarven contraptions.
Fend off insanity, depression and sadistic nobles with copious amounts of dwarven ale.
And above all, learn that losing is fun!

Minimum system requirements: 

~100MB Disk Space, 512MB RAM
Core2: 1.4GHz or higher
Pentium 4: 3.0GHz or higher
Athlon: 3000+ or higher