Fax machines will never die no matter how they are mocked. It simply is the easiest way to send documents with private information and it's fast. At least we have e-faxing now to receive documents.

Dildoes and pocket pussies

fax machines, both in Germany and Japan.

IPv4.

IPv6 became a recognized standard by 1998.

EDIT: https://www.google.com/intl/en/ipv6/statistics.html#tab=ipv6-adoption

Nearly 30 years later, and less than half of the connections to Google are via IPv6.

Steam engines.

The vast majority of our power comes from making something really hot and boiling water. Coal plant? Oil plant? Gas plant? Nuclear fission plant? Geothermal plant? The grand holy grail of energy production that would be a nuclear fusion plant? All steam engines.

Yes, unbeknownst to everyone, this is what a steampunk society realistically looks like.

Microsoft Windows

It surprises me how many system utilities I use that are older than I am. I am currently initializing a disk on a cloud server with an application that was written when Ford was the US president.

The Wheel. We should've graduated to antigravity by now, don't you think?

I'm surprised nobody mentioned jack plugs yet. Basically unchanged since 1877 when it was invented for phone switchboards, roughly as old as safety pins or modern hairpins (give or take a few decades)

Car thermostats for the radiator. You don't want the coolant flowing when the engine first starts, because it will run like shit. So you have a cylinder filled with wax that expands with heat. That controls a valve to set the flow of coolant. Low tech, works fine, no particular reason to change it.

There's a used bookstore near me that has the oldest cash register I've ever seen. It has keys like a typewriter, and makes the most satisfying "ka-ching" sound when it opens. They always use it to add up your purchase and print a receipt, even when you're paying with a credit card. But I always try to bring cash when I'm there so that the drawer gets used. (And also, y'know, screw credit card companies taking their cut.)

I know that's not really "in widespread use" today, which is probably what the question meant, but that was the first thing that came to mind for me.

Fax, still in official use in Germany.

Air traffic control still uses floppy disks, windows 95, and a plastic board of paper tag numbers to keep track of shit instead of a computer.

The IRS still use COBOL.

SS7, part of the old ass 2g and 3g networks

Steam turbines.

Floppy disk.

Pagers

It surprises me how little stick-built houses have changed in the last 50 years or so, at least in the USA.