This is a great thread! I help run a night which is exclusively one-offs, so lists like this are super useful to me.
In addition to the games others have already mentioned here, many of which I love, here some satisfying low-prep one-shots from my arsenal:
I echo the recommendation for Lady Blackbird. Not only is it ideal for a one shot, it is the best game bar none for introducing players new to the hobby to roleplaying.
Keys mean players are mechanically rewarded for roleplaying in ways specific to each character.
I’ve run Lady Blackbird at Games on Demand at Gen Con and for my parents so they understand my hobby. Each character sheet has the rules at the bottom. Everything is self-contained.
It’s a marvel of doing a lot with a little.
I always have a deck of Archipelago cards on hand and am thus prepared for a good time with zero notice.
What are the prepared scenarios? Are they the same for the most part or do you have a specific for each system?
Most of my off-the-cuff scenarios these days are boiled down to a clear hook and a horrid truth (but then again, much of my material is horrid investigative fare)
A typo that makes me want to get hacking. 
But is it about international espionage during the Cold War, or the management of Simulations Publications, Inc. from 1969 to 1983?
My own deck has yet to be deployed, I think the time has come.
I play mostly at conventions.
InSpectres: Takes me around 45 minutes to play, including pitch and character generation, where I introduce rules as we go. We usually play this twice, where I can introduce even more rules.
This is Pulp: Takes about 70 minutes to play, including pitch, character generation and world generation.
Dungeons and Bananas: Takes about an 60-90 minutes to play, including pitch, character generation and world generation.
All these are based around banana challenges, where the game master creates challenges where the players describe their way out of the situation. “How do you overcome the locked door, using a banana”.
Kagematsu: Convince the lone ronin to defend your village. Takes about 3-4 hours to play.
The Coyotes of Chicago: Playtest document, where we have three game masters and one player, and each game master have their own agenda and elements to introduce. Takes about 3-4 hours to play.
I rarely play Kagematsu because I refuse to play it unless a female is playing the samurai according to the rules. I mostly play Coyotes, if the participants wants to play something longer, and I want to show them something unusual.
The Murder of Mr. Crow: Takes about an hour to play. A storytelling game that takes place in a Sherlock Holmes inspired environment where the detective is just about to tell everyone involved who’s the murderer.
Imagine A non-conflict storytelling game that takes about an hour to play, including pitch and rules explanation. A game that makes people emerged in the story, and you’re only allowed to speak one sentence and then have to wait until someone else have spoken. Based around the East-asian narrative structure kishotenketsu.
Why play longer games when you can play several ones (or the same one) during the same time? 
I ran a short-lived Monster of the Week campaign awhile ago, so I have around four scenarios for that game I’ve already written up and can play without prep:
For the other three (Troika, DCC, Knave) I have a large collection of old D&D/AD&D modules as well as zines and modern modules, including many of the DCC modules. (I also have some old maps and notes for scenarios I’ve created but since I have an over-supply of zines and modules I’m usually excited to try out one of those.)
I’m curious if you have any particular thoughts/comments about your experience running this as a one-shot.
Re: The King Machine, there’s an “opening crawl” you can read to get players quickly up to speed with the setting; the rules are relatively short; character creation is quick; and a number of tables are provided to generate aspects of the world. Whoever runs the game will need to read the rules ahead of time to fully grok them, though—you can’t just take out the book and go.
Lasers and Feelings and Psi-Run are great.
Havoc Brigade is a blast for in person games. You play Orcs (and a bunch of goblins in a trench coat) trying to infiltrate a castle…farcical hilarity ensues.
Grant Howitt’s one page RPGs are delightful.
Honey Heist is great, but I also played a session of Goat Crashers using the Tearable RPG system and I think it was the most fun I’ve ever had with a game
Hey, great recommendations!
I tried searching for Dragons and bananas but didn’t really find much. Do you have a link?
So some of my favorites are listed above already, but I’ll add one that’s missing and add some more detail about why they are in MY kit.
For the Queen: I absolutely love this game. It’s a game where we learn about the individuals traveling party on a quest as the game progresses. The game IS the character creation as we learn about each other by answering very pointed questions (presented by the cards/game rules). Another reason I love it is that the entire instructions for the game are on 20ish cards you can pass around the table (the instructions tell you to do this) and start play. The act of passing around the cards is also a warm up to get use to reading a card and talking. The actual mechanics are real simple, and pretty loose. You can model good behavior (like asking questions for further detail) or pause to answer anything that is unclear. It definitely helps to have a facilitator who has played before to help model good behavior, but is also not necessary. It’s very re playable even though the same questions can come up, what has already been answered, who answers, and how they answer vary the game dramatically. The fact I can just pull out the deck of cards or spin up a new Roll20 instance makes it a less than 5 minute prep for both online or in person gaming.
Dialect is a game about a language and how it dies. Yes it’s a language building game, but it’s also a game where you learn about this dying community through the lens of several of it’s members. I think the game is well structured and does a good job of guiding you through play. It is a bit melancholic as it is known going in, the language (and community) will die out. That doesn’t mean all the characters will die, they might be assimilated into a larger population or something else. There can also be individually heroic stories/arcs, but the undertone will have notes of loss. The only prep is deciding what backdrop to use (which usually takes about 5 minutes) and setting up the actual table (about another 5 minutes). So a 10 minute prep game (both for in person or online, again a Roll20 module) makes it one of my go to games for no-prep one shots.
Reunion is a Descended from the Queen game, I’ve been working on with Jenn Martin and Andy Munich. It’s about a group of people who used to be real close, but have been a way for a while and are coming back together. It’s not released yet, so it doesn’t really help anyone here at the moment. But it shares many of the benefits of For the Queen, plus I get to see my own game being played (which can help to improve it).
Fall of Magic is a game where you travel with a group on a quest and learn about them through vignettes on the journey. The quest is that magic is dying and with it the last magus, you journey to Umbra, where magic was born. Maybe to restore it? Maybe to watch end? While it matters somewhat what the end is, it’s more about the Journey than the destination. The physical map and pieces are absolutely gorgeous (the online version is very pretty too, but lacks some of the heft and satisfying physicality of the physical version). I am way more likely to play this as a substitute game online than in person, not because the play is better, but because UNLESS I am playing at my house, I am just unlikely to have my copy of the game with me. Sometimes at cons or other places there will be a copy around to use, and then I’ll use it. But I’ve discovered, for myself, that while the physicality is beautiful and wonderful, it also makes it a lot harder to just toss it in my bag of gaming stuff and carry with me.
Dead Friend by @oh_theogony is a wonderful 2 player game where one person plays someone trying to summon the ghost of someone they care about, the other person plays said ghost. The structure of the rules guides you fairly well through the whole process. The rituals of play also help give the game a great atmosphere. I’ve only played once, but now keep a couple copies printed out in my game bag just in case.
It may be obvious but all of the games on my list are facilitated rather than GMed. All but Fall of Magic (which to me is more of a three shot game to get to the end), are explicitly designed for one session of play. They don’t have character creation, or rather the mechanics have you learn about the characters over the course of the game, rather than per-defining a bunch of stuff before hand.
Ops, translated the name from the Swedish one, and forgot that the game is called something else in English. I updated the original post with links to all games.
Here’s a slightly different take on the question…
I keep a dop kit full of pokerchips, dice, pens, pencils, playing cards, notecards, card holders, and a microfiber cloth. If I can find one I like (and will fit), I may add something a tarot deck to help with inspiration.
I also, keep a number of clipboards and binders around with notes, printed playbooks, NPC’s, and etc. I keep one per game system (it wasn’t planned that way, it just worked out).
And an old Kindle Fire chock full of PDFs.
Thanks, that’s very kind. Looking forward to playing a lot of those
Two games I haven’t heard mentioned yet:
Better as a long term game, but still makes for great one shots! (I’m developing a “kit” for particularly easy play, which eventually I’ll have thrown together.)
A nice, light-hearted GMless detective mystery, a la Agathie Christie. Very British.
A GMless storytelling game, easy to learn and play and always satisfying.
Although I have not played these, I have seen from a very good source on rpggeek, the following suggestions:
Juggernaut
Dirty Secrets
Unheroes
Here are some games that technically could be played for more than one session, but in my experience are perfect for one-shots and no more…
Dust Devils
Danger Patrol/Pocket Danger Patrol
Hollowpoint
I also don’t think anyone mentioned Trophy or Trophy Gold. Trophy Gold looks awesome!
In addition, for a one shot I forgotTiny D6 games (Tiny Frontiers, Tiny Dungeon, Tiny: Mecha and Monsters) are great if you want a simple one shot dungeon crawl / pick up and deliver / kill the kaiju game. The huge number of mini-settings with hooks and villians works great for a one-shot. It fails in the character development and feels areas though.