Ask HN: What's the most widely deployed but least used technology?

I've just been using a terrible old Acer Travelmate laptop and I noticed the little swipe-to-unlock fingerprint sensor between the buttons on the trackpad.

It strikes me that there have probably been 100,000,000 of these sensors made and I can't imagine the tiny fraction of them that have ever seen any use.

What's the most widely deployed, but least used, technology? Hardware or software. Go!