Some of you may have heard of Minecraft. Well, Dwarf Fortress is what Minecraft was based on.
http://bay12games.com/dwarves/Its AI is absolutely staggering. It generates a world, from scratch, with erosion, mountain formation, caverns, volcanoes, civilizations, trade...it's just incredible the amount of detail that goes into the background of the game.
You can play it in two modes, "adventure" mode, which is very much like a classic roguelike game (Angband, Moria, Nethack, etc), or "fortress" mode, which is the one that Minecraft is based off of. So far, I've only played fortress mode. If I want a regular roguelike, I play Angband
http://rephial.org/ ;-)
In Fortress Mode, you pick a place on the world where you want to build a fortress, equip your seven initial dwarves, and start digging. And building. And making beds, and crafts, and doors, and furniture, and weapons...
It's an incredible sandbox, and the kinds of things that you can make with it are vastly varied, and very cool. My latest fortress has been running for about 7 IG years, and has tapped a magma vent to power its forges, smelters, and glassmakers, and is churning out steel armour and weapons just as fast as my dwarves can make it. I've dug out the housing for a set of pumps that will bring the magma up into a giant holding tank at the top of the cliff my fortress is delved into, so I can dump it out on invading goblins any time I need to. After a number of immigration waves, I'm up to (I think) 142 dwarves, and the Mountainhome is about to promote us to a Dukedom.
If you like building things, and can handle the steep learning curve (following one of the various tutorials is pretty nearly essential), it's an amazingly fun game.
