The famous (or infamous, depending on how you wanna look at it) Google Cardboard.
I remember purchasing this flimsy looking thing about two years back. VR was just getting started but already bold claims were being made about how it was not only possible to experience this using a plain old smartphone, but also the only additional gear required was a $3 foldable cardboard cutout!
At first, it was a laughable ideology, but out of sheer boredom, I recall doing some light online research later that night and was literally shocked by the results. Apparently, it really "was" the real deal. And for only $5 (including postage and package). How can this be? I pretty much ordered it there and then. It took three days to arrive and I was genuinely excited (as well as extremely curious) to see the results for myself.
I'd had the foresight to download several apps on my phone beforehand (one of which was a rather nifty "roller coaster" demo. Old news now, I know, lol.) and was all set to go. It arrived as a cardboard sleeve, to which I soon realised was the actually product! The letter "was" the VR unit! I was blown away. And yet the best was still to come...
As, yes, it did work. And rather well, I might add. Apart from the glasses itself being a little droopy around the front, given the weight of the phone slotted inside, the effect was otherwise perfect. I tried out this train demo, another "cinema" theatre simulation as well as a basic shooting game! The point being for that particular price point, I'd never been so astonished by anything like it in my entire life.
$3 very, very, VERY well spent. :)