DIVE: head-mounted smartphone immersive VR for €50

Your smartphone gets strapped in front of your eyes with a bit of optics inbetween, apps display side-by-side 3D and use the phone's gyro and accelerometer for basic head tracking, and voila. Comes with SDK, some apps already available.

(Sigh. I built a mirror-based spectacle-like contraption 25 years ago and had real 3D (well wireframes of some simple geometric shapes) on my Amstrad PC's greyscale EGA monitor, why didn't I think of this?)