This isn’t really a list of individual things I’ve learned on Twitter but more like resources people on Twitter have pointed me to. I keep them bookmarked for when I need to learn what they have to tell me. All are free to use or at least to try. And some of them are very obscure.
The Stanford Geospatial Model of the Roman World: You select where in the Roman Empire you’re starting from, where you’re going to, and how you’re traveling (e.g. walking, horseback, carriage), and it’ll map out a route for you and tell you how long it’ll take. The tools is specific to the ancient Roman world, but data can easily be extrapolated for other locations or fantasy worlds by approximating the geography and distance covered.
Floor plan creator: Need to remember what your character’s house looks like or just waste endless hours procrastiworking on something fun? Floor plan creator is for you. Behold my loveliness! Note that you can only print with a paid plan, but screen captures are free.
Murderpedia, the encyclopedia of murderers! Described as “the largest database about serial killers and mass murderers around the world.” I mean, I don’t know what you need that for, but if you do, it exists.
The Urban Slang Thesaurus: For when you’re trying to be cooler than you actually are. Just be careful to make sure you’re using the term correctly (and that it’s from this century).
Figurosity: More for artists than writers, but I often find it helpful to have a reference image when I’m trying to describe something. Figurosity shows different body types in different poses.
Vehicle History: This site has specs with interior and exterior pictures for many models of cars and trucks, so if you need to know what the dashboard in a car you could never afford looks like, drop on by.
Working with the FBI: No, really, I’m a writer! The FBI says: “if you are a writer, author, or producer who wants to feature the FBI, we may be able to work with you to create an accurate portrayal of the Bureau. We’ve been doing it since the 1930s.” Hit them up.
Legal Fiction: Similarly, if you need a legal question answered, try this FB group. Just be sure to tell them what jurisdiction your book is set in. Laws vary a lot from state to state. (US-centric)
Comparing Heights: A useful tool for when you’re trying to figure out how your 6′ 4″ MC’s mouth lines up with your 5′ 0″ MC’s mouth. Hint: it’s not good!
Always Up-to-Date Guide to Social Media Image Sizes: This will tell you what dimensions to use for various post types on various SM platforms. The “always up-to-date” part is in their title, but every time I go to check, it’s been recently updated.
Online Etymology Dictionary: Find out when a word came into usage, where it came from, and whether its origins are problematic.
Atlas Obscura: Want to know what tourist traps or oddities are in your character’s vicinity? Atlas Obscura has an interactive map covering the entire world that highlights not just the obvious sights, though the obvious ones are on there too.
British Titles of Nobility: Scroll down a bit to get to the Table of Contents. There are great resources here for order of precedence, how to refer to various nobles, etc. Invaluable if you’re writing British royalty and want to get it right.
Those are the strange but handy resources I’ve bookmarked over the years on Twitter. What else can you suggest?