This one wins the spirit of the thread just for it’s simplicity.
The Sunbeam T20! No buttons, just drop the bread in, it knows what to do. I got frustrated with the $5 hardware store specials.
Through a process of deductive reasoning, I surmised that it was extremely likely that some sort of “obsolete” though completely reliable vintage device must in fact, due to certain laws of the universe, exist. Made back when the chrome was thick, and you know. A few clicks on the internet and there it was! They probably keep one at the Museum of Modern Art. Raymond Loewy.
I nominate the adjustable competition style kettlebell. Others have made adjustable bells before, but the competition style has the weights inside of a hollow, competition sized shell.
This makes them easier to store, and all of the movement patterns remain the same, no matter the weight. The one from Bells of Steel has .5kg and 1kg plates, so you can go from 12kg-32kg in very small increments (41, actually.)
12-32KG Adjustable Competition Style Kettlebell - Bells Of Steel USA
Dryer thermal fuse. If the dryer gets too hot (by a clogged vent, for example), the fuse burns through and cuts off power to the heater. Simple thing that prevents house fires. Cheap part. Easy to replace.
I have two appliances with the thermal fuse jumpered out. A George Foreman type grill and a coffee maker. I don’t leave them unattended while in use. I’m not saying it’s a good idea.
I’ve been kinda trying to figure this out myself. I figured hydraulic, never thought of magnets.
My vote - Kleenex designed a box that works very well by just putting it on a towel rack.
I don’t think it’s either - it’s just friction.
Is that a response to me beowulff? It’s about the shape of the box.
About the toilet seat.
We have one, and the mechanism just seems to be a viscous hinge.
A vicious hinge? On the toilet? What? Oh never mind - Emily Litella.
They’ve taken it to the next level and now the rear view mirror (and side view mirrors) can auto-darken based on the intensity of light hitting them. Pretty spiffy.
ETA: not sure why this post is appear as a reply to Chronos when it’s a reply to Burpo.
Chronos does that every once in a while when he’s feeling lonely. 
I’ll say. I don’t like when the back-up camera is on the rear view, but that’s my wife’s car and I don’t have to drive it, nyah, nyah, wifey.
It darkens faster than the speed of light! If only Einstein was around to see it.
I have a SawStop and have triggered the safety mechanism twice. One time the blade nicked my alumninum miter gauge fence and the other is when I wasn’t thinking and tried to cut some thin sheet metal.
I want to say the replacement brake is about $80 (or was) and the new blade costs whatever you want to pay for one, but the ones I buy are $20 - $80.
Hm. I guess the price has gone down. Unsurprising, now that they have more economies of scale.
I saw that at the grocery store. I was hoping it would hold commercial-sized tri-fold paper towel packs, but alas, no, wrong dimensions. Just super expensive $4.49 for 60 sheets. (It’s intended for countertop, but towel rack is an interesting idea.)
Rotary Dampers
Which are essentially viscous hinges.
Huh. Intended for countertop? I guess my mom came up with the idea of dropping the angled part into a towel rack. Then it stays stable and the towel is dispensed from the bottom.
Here’s a seemingly trivial example that’s a good illustration of how technology sneaks up on us: flashlights! I mean, what’s to improve on? You stick some batteries into a tube, press a button, and you get light!
You don’t have to be very old to have flashlights from that era. I still have a couple of traditional Maglites, which were known for their build quality and quite expensive.
I recently got a couple of new ones (not Maglites) because it’s always useful to have flashlights around the house. New technology and some design thought have made awesome differences:
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The new ones are pocket-size – less than 6" long, about the same length as my mini-flashlights that take two AA batteries but thicker because of the robust Li-ion battery – but the LED beam is at least as bright as my traditional big Maglite with four D cells.
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The beam is adjustable from wide to very narrowly focused and distance-penetrating
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It has a heavy-duty rechargeable Li-ion battery claimed to provide sustained illumination for 17 hours. It has four little blue LEDs indicating charge status. It’s rechargeable via USB.
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It’s rated IPX6 level waterproof
Maybe a kinda silly example, but it’s amazing how far these simple but often essential devices have come. It also has a range of brightness settings and strobe/flashing modes, achieved by successive presses on the power button. So how do you turn it off without cycling through all the modes? Another thoughtful and simple design choice: after 8 seconds in any mode, one click turns it off.