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In the basement, where the electric meters (and ancient fuse boxes) are, overhead there's an oversize cermic socket with a string-pull switch, used only when reading meter or hunting for scraps of wood, holding a three-way Mazda-type bulb which probably burned out its 200 watt filament but still works on 100. It was there when we moved in August 1979. The heating system is a lot older than that.
Plus we have one of those lovely mono FM radios with the 4" speaker with a 5 lb. magnet on it made by KLH in the early 60's, still works beautifully; and our big stereo, much newer, drives KLH speakers I bought just after they came out c. 1963 to go with a Scott kit I built then. These KLH speakers sold for around $50 each, each box containing two of those 4" speakers with the huge magnets and the low-pass network with allowed great sound very inefficiently but very cheap first cost.
And how about a jar wrench, still as good as any, using rack-and-pinion. It was a wedding present to my parents in 1934, or perhaps was my maternal grandmother's (she died in 1936.)
Once in a while, I still hook up my ColecoVision game console from 1983. Everything still works, which is amazing. The oldest thing I still use regularly is a 1st generation Bose Wave CD from the late 90s. I ordered it from an infomercial after a night out with friends in college - thanks beer. It cost around $500, which I could in no way afford at the time, but since I still use it and it still sounds good, I feel a lot better about it now than I did the morning after I ordered it. It often gets used as a killer speaker for my laptop when I watch movies, since I haven't bought a new CD is years.
I still have my original Atari 2600 system and a bunch of games - finding a TV that accepts the inputs can be an issue. I'd love to play an old ColecoVision - do you still have Donkey Kong?
Also, we have a Bose Wave CD player - but I think we got it in the 2000s, not the '90s, so i'm still trying to think of something that I still use from the '90s or '80s.
I have a Sony Trinitron (one of the flat screen CRT TVs) also from the late 90s that I use for vintage gaming systems to make it easier. The ColecoVision was pretty impressive at the time. Moving up from the 2600, it really did seem like it was "arcade quality." Donkey Kong, Donkey Kong, Jr., Zaxxon and Popeye were the games I still own that my friends and I mostly played. DK Jr is the one that has probably aged the best. Coleco had some kind of weird distribution agreement with Sega and Nintendo, so you get a small but above average library with the ColecoVision. If you like vintage games, you should check out some of the Angry Video Game Nerd videos. The Nerd videos are hilarious, but very profane and NSFW. They are on YouTube and Cinemassacre.
I'd have to argue with restricting to modern: how about wheel, wedge, screw (which combines them), metal alloys, etc.
If you insist on "moden", then incandescent light bulbs of the Edison derivation; not replaced because nobody seems to make affordable bulbs of 3-way with lumens as high as the 50-200-250 watts incandescents.