Maybe this is dangerous, and maybe you've already heard of it, but I present...
Dwarf Fortress, quite possibly the most complexly thought out game ever created. The graphics are basic, to say the least (though if ASCII doesn't take your fancy there are fan created tilesets available) and the learning curve is atrocious... but if you can get to the end of Wikipedia's article without at least feeling intrigued then you are no true nerd.