Google Cardboard

Nothing more than a cardboard box with two bi-convex lenses in which you can fit most smartphones. Simply download any VR app and insert the phone. An instant, low-cost VR experience.

Price: USD 15 to 35
Availability: Plenty, available through online retailers or local auction sites.


Pro:
* Very easy to setup
* Low cost
* Instant fun
* Works with all smartphones (iOS, Android)
* No cables, no other hardware needed,  easy to use everywhere

Con:
* Smartphone buttons might be pushed by box (if not perfect fit)
* No input devices except for simple magnet trigger
* Uncomfortable
* No perfect fit for either phones or heads
* VR applications limited to resolution, speed and storage of smartphone.

Details:
* Simple headmount for smartphones
* Two bi-convex lenses (25mm Diameter | 45mm Focal Length)
* Magnet trigger as basic command input device
* NFC tag to identify the VR headset (smartphone can be setup to launch app automaticaly when inserted into the cardboard.

Links:
https://www.google.com/get/cardboard/