Google wants "everyone to experience virtual reality in a simple, fun, and inexpensive way" with its Cardboard project. It's a template for making a "no-frills" cardboard enclosure -- such as the expertly crafted one above -- that houses Android phones to provide a basic VR experience.
This obviously won't compare to a newer-model Oculus Rift, but that's the point: "By making it easy and inexpensive to experiment with VR, we hope to encourage developers to build the next generation of immersive digital experiences and make them available to everyone."
Besides instructions for the headset itself, Google has provided an experimental VR Toolkit and documentation for developers. A fun weekend project for the rest of us, perhaps.