Also D&D - A Bag of Holding. Not unlike a TARDIS, it is much larger inside than outside. Holds approx. 64 cubic feet of stuff but always weighs 15 lbs.
In Paladin of the Lost Hour an unassuming pocket watch is all that keeps the world from ending.
If you haven’t seen it, this is pretty much the entire premise of the Syfy show Warehouse 13 – a giant warehouse where the government stores artifacts with remarkable properties. I have to imagine if must have been inspired by that giant warehouse shown at the end of Raiders of the Lost Ark. It’s been a while since I watched the show so I don’t recall any artifacts (and I only ever saw it on Netflix, which only seems to have ever had the first two seasons), but I’ll crib from Wikipedia:
The Pick Of Destiny (from the movie of the same name) bestows superhuman abilities on stringed instruments to anyone who wields it. According to Ben Stiller, the pick was fashioned from a tooth from Satan.
All suggestions are welcome of course (and there are some good ones here), but what I had in mind were artifacts that were unassuming both in basic presentation and operation. A Bag of Holding is an excellent example: it’s pretty much an ordinary bag, just a lot more useful. An unaware bystander watching someone use a Bag of Holding might take quite some time before going wait a second (say, after an anvil is inserted). On the other hand, The Luggage isn’t as good an example since no one could fail to be surprised after it sprouts a dozen or so legs and eats a bad guy.
Numenera is probably what planted the seed of this thread, as I played through Tides of Numenera recently. I recall an ordinary-looking cup that would heat the contents of (nearly) any liquid poured into it, but without any obvious means of power or otherwise.
Mary Poppins’s carpet bag appears to be pretty much the same as the Bag of Holding, and it predates D&D.
And Mary Poppins’s umbrella allows its user to, if not exactly fly, at least float through the air propelled by the wind.
In R. A. Lafferty’s “Bright Coins in Never-Ending Stream” a man has a small coin purse which is never empty. Unfortunately, while once it gave out gold eagles ($10 coins) it has decreased the value of the coins it now gives out - and it’s small, so you can only fish out one coin at a time.
I love the Immovable Rod. Get two of them and you can literally climb up air.
GURPS Warehouse 23 was a supplement for the classic 3rd Edition of the Generic Universal Roleplaying System. It was very explicitly inspired by the ending scene of Raiders of the Lost Ark (what else was in all of those other crates?!). The book has information on the Warehouse, the staff, security measures, and so forth, and a number of the artifacts stored there. It’s got most of the big, obvious ones - Ark of the Covenant, Spear of Destiny, and so forth. It’s got some big, custom-made ones. But it’s also got quite a few along the lines of what OP appears to be looking for. Some of my favorites:
The Spear of Destiny: There are a number of suggestions of what it is and what it does. One suggestion is that it’s a perfectly ordinary-appearing Roman legionnaire’s spear, remarkable only for how well-preserved it is. As for its power - one suggestion is that it inspires the holder to Greatness. Great Good or Great Evil - the spear is indifferent. It just helps you become the Greatest, most extreme version of yourself. It’s an ordinary-looking object with a very subtle but powerful effect.
The Memories of Michael Perry: An ordinary, slightly shabby wool cap. It contains all of the memories of an young man, named Michael Perry, who had a very hard life. Wearing it gives you his memories, as if you had lived them. It literally gives you a different perspective, and forces you to walk not just a mile in another man’s shoes, but walk his entire life.
Astro Globs!: An early 80s cartridge for a popular home video game system. The game itself is a sort of cross between a digital lava lamp and Tetris. It’s oddly relaxing, and just difficult enough to be a challenge for you (yes, you, specifically you, whoever you are) to master, without quite being difficult enough to be frustrating. And it’s utterly addictive. You can easily spend hours playing it non-stop - or days. Without sleeping. Or eating. Or drinking. If you’re not forced to stop by outside intervention, you’ll eventually die of dehydration. And if you are forced to stop, you’ll go back to play, because it’s just a really fun game. Just one more level… It’s got a ridiculously advanced adaptive AI. Effectively, if you play it long enough (and you will…), it creates a comprehensive, perfect, deep neurological profile of you. Who is it being transmitted to? And why?
There’s the tents in the Harry Potter series. Look like an ordinary, average sized tent from the outside but generally a multi-room furnished suite inside.
And her bottle of cough syrup from which she poured three teaspoons full, each a different color.
The TARDIS, at least at the beginning of the show, when police boxes were common.
Jesus found some mud that restores sight.
I loved when I first saw that as it opened up a world of possibilities with that. The other one I like is if you stay at an Inn and the door opens inward, you put the rod on the inside and activate it and that door won’t open without so much force, it will wake you up. Same for the window, or through it’s loops if it opens out. If you have enough, you could make a platform float in air to hold a tent as a safe place to stay. It really opened up a lot of ideas for me!
Another show that had mundane items but they do things is Friday the 13th the TV series. The items were cursed in some ways, like a surgeon’s scalpel that would make sure you succeed in saving someone’s live, providing you took a life with it. Or boxing gloves that would let you win while your shadow beat someone to death. I had to look those up. The one I do remember was the garden mulcher that when you put someone into it, they turned into money, approximating what they were worth. If it’s used for mulching, though, it works and doesn’t reveal what horrific thing it can do.
Thanks for the interesting conversation!
Two more from Ringworld. The Variable-Sword, which was just a simple extendable wire with a handle. Held in a stasis field, which made it absolutely inflexible, and therefore able to cut through anything. Except the second item.
The General Products Hull. Completely invisible, and unaffected by any amount of impact or energy except antimatter. Among other things it was unaffected by was impacting the Ringworld surface at 770 miles per second. Though everything inside the hull had better have some means of protecting itself…
The Ringworld structural material was not quite invulnerable, though had a tensile strength on the order of the binding energy of a nucleus.
Since we’re allowed to stretch the definition a little bit - and since no one else has said it - Sonic Screwdriver
I was reading a story with a similar immovable object…guy was running away from a big areal predator and he dropped the rod behind him just as it came swooping in to snatch him up. It goes without saying that big bird didn’t have a good day.
The Known Space short story *Flatlander *has a wine glass with a tiny teleportation receiver in the base, hidden by distortions in the glass. The result is a never-emptying glass of wine.
that goofy guy with the ordinary-looking shoe with a secret telephone in the heel.
Two Phone Booths
One in The Blues Brothers, could fly and save you from destruction.
One in Bill and Ted. Time travel. Enough said.