This game is so good the developer actually lives off monthly DONATIONS.  And I even donated $50.  And I'm poor, but this game is very interesting.  He hasn't updated the version in a while but it's a great game and should only get better.

There's two modes.  The primary is Dwarf Fortress where you take a group of dwarves and build an enterprise, mining out a fortress somewhere in the world (Which you pick, and generate the worlds, all having a history of their own) and try to attract more dwarves, or do whatever you want really.  It's hard to explain how fun/good this game is until you play it.  So I'll just leave you with the links.

But just for a taste, you can build levers and machine parts and create all sorts of things, from wind/water mills to a pipe pumping lava from an underground tunnel.

The other mode is Adventure mode where you create a single character and go around doing stuff- this is pretty sparse but can still be fun.  Especially if you get them to some crazy levels of combat skills and rip peoples arms off or something. 

All the graphics are ASCII and it can take a while to get used to it, but believe me it's well worth it- also there's different graphic patches made by people if you need more then that.  Eventually if it keeps going this game will have a war system and much, much, more.

Here's a random quote I noticed just from grabbing one of the pictures:

"An innocent link on Penny Arcade hooked me on Dwarf Fortress. You shouldn’t even consider trying it if you don’t have massive amounts of time to waste and are in any way curious about the intricacies of dwarven life. If you can spare the time, the rewards are amazing - ASCII graphics, a hugely complex dwarven economy and social system, deadly trap systems and cruel and unusual punishment await!

(Life will never be the same after you’ve spent a couple of hours constructing an elaborate system of channels and floodgates to create a remotely triggered drowning chamber for a goblin that wandered into one of your cage traps, only to realize that the swamps you tapped to provide the water will only raise the water level to 5 or 6 out of 7 and thus not drown the goblin….)"

http://www.bay12games.com/dwarves/

And it's got so much to learn in it- it's own mini wiki-

http://dwarf.lendemaindeveille.com/index.php/Main_Page

We're all gonna be here a while, I can see this game helping many of you through that time.  Seriously, check it out. 

Here's some random pics grabbed from the web.