Until someone builds world peace in Minecraft, we'll have to make do with this: an Atari 2600 emulator running in the increasingly disturbing thingbuilding sim. It's the work of one SethBling, who has quite the reputation for making stuff in Minecraft: he's made a BASIC interpreter, a version of Nintendo's Splatoon, and collaborated on a blocky version of Team Fortress 2.
The video below provides a brief demonstration of how it works. It uses "a couple thousand command blocks" which run the processor, the video and the memory, while also containing the ROMs that were stored on the original Atari cartridges. It can output 60 frames every four hours.
It's pretty impressive, when you consider this was built in vanilla Minecraft, with no mods or plug-ins. Anyone could feasibly make this without tinkering with the core game files, in other words. Check out the general demo below, or if you want something a bit more technical, click on over here. If you want to play with the emulator yourself, here's the files.