Dwarf Fortress (free) is...complicated. More complicated than anybody's romantic status, no matter what they say on Facebook. Understand that when I say Dwarf Fortress flies straight over my head, it's not because I can't abide any game which requires more than adrenaline and a mouse capable of 500 clicks per minute. Also understand that I am not saying Dwarf Fortress is a bad game. It is simply... well, nothing about it is simple. Dwarf Fortress is immense, hugely complicated, insanely detailed, and uses only ASCII characters for graphics.
This is a game which takes the processing power most modern computers use to model the bounce physics of Lara Croft's... bullets... and uses it for the raw game engine. The game runs in a DOS window, with everything represented by characters, mostly punctuation marks and "line graphics". Depending on what you're doing, this already small piece of real estate may be subdivided into smaller views. It isn't pretty, but, man, is it deep.
In Dwarf Fortress, you control a bunch of dwarves who build a fortress. This statement is about as useful as "In World War 2, some people had a fight." First, an entire world, with thousands of years of simulated history, is generated, because you're likely to be using the same world over and over, as you try to build forts in different places. Then, you and a band of seven dwarves set out. Of course, you need to equip your dwarves first, and train them, and both involve huge lists of options and items, including everything from kittens to anvils. Make the wrong choices here, and you'll end up being unable to do anything. Of course, what choices you make also depends on the type of region you've chosen to settle in, as there are huge variations in the type and quantity of resources.
Once you're done with this (you can also accept a default mix of professions and gear), you go on to your first settlement, and that's where things really get hairy. (Dwarves, hairy... get it?) In some ways similar to a typical RTS (in the same way a tricycle is similar to an M1 Abrams), you designate zones for various activities, such as mining or chopping wood, for fishing, gathering plants, piling wood, making beds, and much, much, more, then designate areas for different purposes, such as bedrooms, and absolutely nothing is simple or without options, the Dwarf Fortress wiki article on bedrooms is almost 3000 words long.
A world this detailed can inspire great devotion. One enterprising player managed to use Dwarf Fortress's system of gears and pressure plates to build a functional, if very slow, computer. Let's just say I didn't get that far. My dwarves did a little mining and foresting, then ended up milling around, "Idle," and I could not figure out how to get the little hairy buggers to get back to work.
Dwarf Fortress is in no way a bad game. It's an amazing game. But it's a game which requires dedication and patience. It has a very steep learning curve and an unfriendly interface that hearkens back to 1983 (despite being first released in 2006), where the same keys do different things on different maps. Dwarf Fortress is not just a game. It's a lifestyle.