Google’s developer conference, Google I/O, is famous for its swag: phones, tablets, other gadgets. Last summer,  Google Android boss Sundar Pichai gave out a box called Cardboard.

Cardboard turns any smartphone into a “virtual reality headset.” Fold the box. Slip the phone in at the end. Peer through the darkened box to a 3D app on your phone. Voila! Virtual Reality.

The conference was held shortly after Facebook bought Oculus VR for an astounding $US2 billion. Oculus is building a virtual reality headset that renders 3D graphics so realistic, you feel like you’ve stepped into a Star Trek holodeck.

Cardboard was clearly a Google joke, a techie gesture that said, “Hey Facebook, we’ve got a VR headset, too, and ours didn’t cost $US2 billion.”

Flash forward to now and the funny thing is, Cardboard is no longer a joke, as spotted by blogger Steve Parker on Utah People’s Post.

Instead of the do-it-yourself instructions for making your own Cardboard, you can now buy Cardboard from four manufacturers. ($US15 – $US35). Some of these devices come with fancy features like I Am Cardboard’s Eva ($US35), which includes a NFC (near field communication) tag to launch apps. Another, Unofficial Cardboard, says it has already sold 50,000 units, and growing.

There are now over 2 dozen apps available for Cardboard on the Google Play store such as Tilt Brush gallery, where you can view 3D art creations plus various games, tours, and so on.

While there are more apps for Android than iPhone, there are a few Cardboard-capable iPhone apps on the App store, too.

And earlier this week Google released an updated software development kit for Cardboard, to encourage developers to write even more apps. This isn’t just a joke, it’s now a real thing.

Cardboard could be the perfect stocking stuffer for the tech gadget lover in you life.

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