MInecraft.
The game that's not really a game. More like a toy. Digital Lego.
For those that aren't savvy, it's a sandbox game in its purest form. You spawn in a randomly generated, blocky world. the only instructions are 'Press 'E' to access inventory'. It's impossible to play without consulting the wiki. I would normally call that cheating, but this game is very different to other games.
The basic idea is this: Ye get wood(careful now, down with that sort of thing), make a workbench, use the workbench to make tools then use said tools to gather resources and build a shelter before night falls. Monsters come at night. Monsters like Zombies, Giant Spiders and Skeletal Archers. There are things called Creepers, they're basically penis shaped hedgerows that blow up if they get too close. And Endermen. Whovian abominations from another dimension who only attack if you look directly at them. Wouldn't be a a problem if they didn't keep using their powers of teleportation to appear right in front of you.
There are no rules and no real goal. There's an end game involving a dragon fight, but it has little to do with the spirit of the game. You make your own rules. Youe make your own fun. Once you have yourself set up with a secure shelter and reliable food source, you start looking around the landscape and thinking:
I don't like that mountain, I'm gonna craft me some TNT and blow the bitch up.
Or:
Y'know what, I'm going to fill this crevice with lava.
Or:
Holy shit, I can make minecarts and tracks. FUCKIN' ROLLERCOASTER
And it goes on and on....
And before ye know it it's three in the morning and ye should have been in bed hours ago