Perhaps the most famous VR-headset for smartphones: The Google Cardboard.
Already more than 500.000 items sold and counting! Also the cheapest and it's a DIY kit you have to mount yourself. I have tried cutting my own one from normal cardboard. I can tell you, this was not a success. Don't be cheap like me, just buy one!
This VR headset does have quite a few downsides:
You have to hold it, it does not have a head strap, it looks awful and of course the lens quality (plastic) is very different in comparison to the more expensive headsets like Gear VR and Zeiss VR ONE.
I Am Cardboard v2 is really good quality Google Cardboard v2. There are various colours to choose from and the v2 version has the iOS friendly conductive button.
Price: 10-25€ /10-25$ (depending on model and extra's)
Country of Origin: United States