Introducing "Captive Minecraft", a Survival gametype that uses achievements and the world border to great effect. You start on a 1x1 box, and every achievement you get widens the world border by half a block in all directions. "As your captive world slowly expands, you gain access to more and more resources," a post on the gametype's website says. "You'll have no trouble Getting Wood or Taking Inventory, but suddenly making that Cake and riding a Pig off a cliff isn't quite so simple..."
You can download Captive Minecraft here — note that the creators highly suggest you play this with friends. This is what you need to get it up and running:
You need to be running Minecraft 1.8 snapshot 14w18b or later. You can set this up right in the Minecraft Launcher. Edit your profile to include experimental "snapshots" and choose the 'Latest Version'.
Now download the world save, place it in your /minecraft/saves folder and unzip it. You'll be able to access it from your single player menu now.
If you're going to run a server, you MUST have Command blocks enabled in server.properties!
And in case you wanted some tips on how to play this, from the gametype's website: