Dwarf Fortress

Taken from PlayThisThing:

Dwarf Fortress is an amazing game. I mean “amazing” at the level of Sim City and Civilization, as amazing to encounter today as they were when first released. I’m not sure I can offer higher praise.

This is an incredibly intense ASCII game, with two playing modes; Adventurer and Dwarf Fortress. Adventurer mode allows you to generate one character, and go exploring the MASSIVE world the AI generated. (Seriously, the thing took over 20 minutes on a P4 3.0GHz.) The Dwarf Fortress option lets you into more of a strategy / empire building game that is very detailed.

Here is the main site: Dwarf Fortress

I highly recommend reading this article on the user-ran wiki while creating your first world /outpost.

Here is a map of the world it generated for me, 25% of actual size. So neat!