One of the best games tbh.
Link:
http://www.bay12games.com/dwarves/
Wiki:
http://www.dwarffortresswiki.net/index.php/Main_Page
Short Description: You start out with 7 dwarves, lead your dwarves to build a mighty fortress!
Tip: READ THE BLOODY WIKI.
It has a bit of a learning curve and the wiki helps alot.