I've been getting into this game for the past few days and can't seem to put it down. It's amazingly challenging, incredibly rewarding, and extremely complex (though it doesn't look like it on the surface). The learning curve is steep, but I strongly recommend giving it a go.
For those of you who have never heard of it, you lead a group of dwarves into the wild and try to build a self-sustaining fortress, while simultaneously defending it from thieves, ambushes, enraged elephants (the horror...), dragons, and goblin sieges. The fortresses can be anything from castles built on a hilltop to vast labyrinths dug deep into a mountainside. The sky is truly the limit when it comes to your imagination with these structures (and the devastating traps and weapons you can engineer). One of the defining characteristics of this game, though, is it's (lack of) graphical engine. It runs entirely on open-GL graphics that look like ASCII art. Thus, your imagination is truly the most important part of the game.
For a true taste of what is possible with this game, I would recommend reading the sad, epic tale of
Boatmurdered (language warning).
Game can be downloaded here:
http://bay12games.com/dwarves/
Fantastic resource (information on gameplay mechanics, graphical addons, etc.):
http://dwarffortresswiki.net/index.php/Main_Page
Amazing series of tutorials (a link to the graphical update set he uses is in one of the early videos):
http://www.youtube.com/watch?v=koZUS2h-Yzc
So... any other players?