Dwarf Fortress is a roguelike-inspired fantasy world simulator. That is, it procedurally generates a whole world with civilizations, monsters, religions, instruments, individuals, wars and history et cetera et cetera. The player then interacts with this world. There are two game modes, but it is most known for fortress mode in which the player directs a starting population of seven dwarves to inhabit a small area of the world and do whatever. You could say it's akin to Minecraft in that sense, which has cited DF as an influence.

DF has been in development for 13 years continuously, programmed by one person who does this full time via donations. He has said it will be his life's work. It's remarkable for it's unparalleled depth. For example, DF is anatomically detailed down to bones cartilage and nervous tissue, and combat between two creatures is a simulation of the material properties of the weapon being used against said organs and parts. That sort of depth is present in pretty much every facet of the game. Oh, and it's in ASCII. Currently, alpha version 0.47.01 is out.