Dwarf Fortress

Slaves to Armok II: Dwarf fortress really is the ultimate answer to the question of what game you would bring on to a desert island.  Once you can get past the ASCII graphics (although there are many fan-made graphics packs out there) you will find an incredibly deep game.  Before you can play the game generates a world, complete with geography, civilizations, history, politics, mythology.  You can then explore this world in Adventure mode which is quite the typical roguelike style rpg game. You can be anything from a sword wielding paladin to a legendary bucket thrower.

  But the place where Dwarf Fortress really shines is the fortress mode.  You start off by scouring your created world for a good location to begin your fortress, then you select what supplies and what skills your starting 7 dwarves will have, then you name your fortress and set off on your way.  In the beginning you are just digging out farms, bedrooms and workshops but after a while you will be drafting armies, attracting Kings and nobles, fending off sieges of goblins, kobold theives and even gigantic dragons and megabeasts. Also if your fortress eventually failsyou can even go and explore the ruins in adventurer mode, examine engravings of the fortess most historical moments, find artifacts that you yourself created and use them in adventurer mode.

  Honestly the amazing thing about this game is that it can make losing fun.  In one of my games i was completely wiped out by a goblin invasion in my second year, in any other game i would be angry that i lost to such a random event, but in Dwarf Fortress i just said ill be more prepared next time and started another one.  And you will lose. My one complaint with this game is the tough learning curve in fact its more of a vertical line than a curve. But there is plenty of documentation out there like the unofficial wiki which is invaluable to your success.  Some may complain about the dated looking ASCII graphics but i would argue that it makes you fill in the gaps with your imagination.  Also after a while it just becomes like the matrix code. I dont see k's i see kobold theives.

This is also an indie game, being developed by two brothers in their spare time though with how frequent the updates are you wouldnt guess it.  Another problem is that the game is still technically in alpha, and thus there are quite a lot of bugs but that is to be expected.  But the main benefit is that the game is completely free.  In conclusion, Dwarf Fortress is a very dense game and is hard to get into but when you do you are rewarded greatly.