Super fun for folks with an old pizza box lying around and who want a cheaper version Oculus Rift!
Cardboard is a project from Google, to help you make your own! Read about it here, more information taken from Laughing Squid: Cardboard is a simple experimental project by Google that transforms an Android smartphone into a basic virtual reality headset using what amounts to a little cardboard, a couple lenses, and some magnets. The project has uploaded design files for the physical headset and some documentation to help developers code software to use with the device.
Here is the official site for the projcet: https://gweb-cardboard.appspot.com/ with an animation to show you how to construct the box. Have fun!
Note that you do need a phone that will fit so do check.