Dwarf Fortress is a free video game with two primary modes: Fortress mode, in which the player takes control of a group of dwarves and attempts to construct a successful and wealthy subterranean fortress in a mountain, and Adventurer mode, which places the player in the shoes of an adventurer as they wander the world and do battle with various creatures.
The game is implemented in text-graphics, and is donationware, so you can download and play for free, while the developers ask for donations to continue their work.