Built by Google, and presented at the Google I/O 2014, this is one of the simplest, easiest, and cheapest ways to get your foot into virtual reality. You can build your own with a pizza box following the instructions given by Google on their website.
It doesn’t come with a headstrap, so you have to figure out how to attach your own if you want to play games that require a controller. It do comes with a magnet button, that you can use with some apps and games that are ready for it.
There are a lot of places where you can buy the cardboard (if you don’t want to, or can’t build your own):