Carburetors. Marvels of engineering and simplicity that held strong through the 20th-century and are still with us today. They work on mechanical principles found in physics and meteorology, without need for tedious calculations. They operate (almost) flawlessly from idle, to partially-cracked-open throttle, to mild acceleration, to WFO gettin’ it; all with no wires or computers involved. With respect to motorcycles, EFI has only recently (in the last 5-10 years) caught-up to the level of performance and drivability that the carb has offered us. I hope they’re always with us in some way… I love you Carb!
Not an everyday item for me anymore but the boarding ladder on the F-18. A few simple little strips of aluminum and a few rods, but it’s compact, can hold 300 lbs and weighs nothing. My favourite bit of elegant technology.
Also not much used is the desktop phone. A little ubiquitous piece of elegant design that works so well that you don’t even realize it.
I can’t think of a single, tiny part on an F-18 that I would consider mundane or ordinary. Since you have actually boarded one, I now love you and hate you. 
I’m afraid to ask if you flew them. I might explode from jealousy. 
I have one that you can staple brochures and booklets on. Got it at a thrift store several years ago. Haven’t used it yet but it’ll be handy in case I need to leaflet the neighborhood or something.
And I’ll see your stapler and raise you the paper cutter. The old fashioned kind with the heavy wooden base (did you know they’re plastic now?) Align your paper and lower the cast iron arm that holds the sharp blade and presto! As long as you’ve kept enough pressure on the paper you get a perfectly straight edge every time! I also like the “gruuuuuh” sound it makes as the arm is being lowered.
eggbeaters
so few parts and so much frothing.
My penis.
Hands. They’re such a part of us we don’t stop to think about how amazing they are. They’re not specialized at all - they punch, grasp, tear, handle tools, and toss things. Humans are one of the only (if not the only) species capable of throwing a projectile with our hands with any degree of accuracy. Everything in our society is built because we have hands capable of building it.
I am fascinated by making mundane, ordinary connections between thread replies. For example, pairing your item with these descriptions:
Simple as hell. It’s the definition of purely functional design perfection.
So simple and yet they radiate a sense of joy, of hope, of optimism.
One of the oldest, simplest tools known to mankind, yet still one of the most beautiful, elegant and downright essential.
It’s like a little time capsule, I’ve never seen a product with so little design change.
They operate (almost) flawlessly from idle, to partially-cracked-open throttle, to mild acceleration, to WFO gettin’ it; all with no wires or computers involved.
A little ubiquitous piece of elegant design that works so well that you don’t even realize it.
so few parts and so much frothing.
And just for hilarity:
I especially like the old, old ones that are made of cast metal and weigh about 20 pounds.
Slinkiest are also pretty fascinating. I would like to know how do they work!?!?
Since magnets and thermos’ have been mentioned, I’ll add this one: grocery stores. I can buy things at a grocery store that kings couldn’t acquire 200 years ago.
Phonograph records and phonographs. I can understand you you can record a lot of information on a tape, CD, DVD, etc. But it is amazing to me that the mechanical process that creates a record can possibly contain enough information for high fidelity reproduction.
I love the terabyte external hard drive I bought about 6 months ago. I am old enough to remember the days when it would take a roomful of equipment to store that much data. I can now hold that in the palm of my hand.
Also when it comes to human body parts, I think hands are much more awesome than penises in an overall advancement of humanity sense. Though of course we wouldn’t have a humanity to advance without them. 
All that and more: Hi-fi repro is produced by the record groove minutely squeezing a diamond, which somehow creates a tiny electrical current that is specific enough to play back the info once the tiny current is increased by a magnitude. I’m sure Gandalf is somehow responsible. 
I like elevators. And what’s not to like? It’s a box that can carry you up hundreds of feet without effort. And like so many things mentioned in this thread, part of the wonderfulness of it is that it’s such an elegant design. Been around for over a hundred years with relatively few changes. And we take it for granted so much that there’s always someone who complains because the music being piped in is kind of boring. As if that’s not a modern miracle in and of itself! Even the name is great; elevator. A device for elevating. And yes, I know there’s a bunch of folks in other countries that call them lifts, which is a synonym. But ‘lift’ is really quite crude in comparison. “Elevator” has class, it has presence, it’s less boring than lift.
Plus, everytime I summon one I feel like a necromancer using infernal powers to compel a demon to carry me into the sky. 
Photography blows me away when I contemplate all the inventions and discoveries that had to work perfectly for us to master it. Mechanics, chemistry and lenses. Cool. Taken for granted for a long time.
Burpo. I recall reading somewhere (pretty sure it was here on the SDMB) that the “diamond” needles in phonographs didn’t really contain an actual diamond. They were called diamond needles because they were diamond shaped.
Fly rods and dry flies, especially old ones.
I was all ready to agree with you, Peanuthead, believing my Dad had skunked me again, but a quick perusal of Wikipedia says “Diamond” needles were actual diamonds, “Sapphire” were actual sapphires and one version had a sapphire needle with a diamond tip.
The Wiki article is under “Phonograph,” subhead “styli,” and I’m going to read it more in-depth tomorrow as I’m ready to pass out for the night.
Do we have actual truth in advertising, here? The mind boggles.
Fabric. If you visit one of those old-timey farms where you can see fabric go from either cotton to a loom or a sheep to a loom, and then think about how that can happen today and end up in the form of a $3 tee shirt…damn.
I can’t wear plastic boots because they awaken my inner child (not that she sleeps very deeply). The best thing about plastic boots is being able to stomp around in your own personal pools! Weeeeeeeh!
The means we have of transmitting information. We often get people saying things like “but Amerinds don’t look nothing at all like people from the Far East!” - but they’re so used to seeing pictures and to seeing people of different ancestries that they just can’t put themselves in the situation of an explorer faced with nothing but referential descriptions.
Looking around me and seeing people of all kinds of different ancestries, in almost any corner of Europe, as made possible by modern transportation and desirable by modern communication. My Latvian boss is shocked by how many “non-Swedish-ancestry faces” we see in a tiny Swedish town; Latvia’s minorities are “people from other parts of the former Soviet Union”, similar to how it was in a lot of Europe a few decades back - different cultures, not-so-different looks.