Links

—in no particular order.

maths

Logic Matters: Peter Smith's blog on mathematical logic. Lots of resources. In particular, his study guide was very useful for me when I was an undergrad.
Model theory of arithmetic: Lecture notes of a course on the model theory of arithmetic by Tin Lok Wong.
Reverse Mathematics Zoo: Describes the relation between various subsystems of second-order arithmetic. An online viewer can be found here.
Zoo Viewer: Visualizer for various "zoos": computability menagerie, reverse mathematics zoo, enumeration degrees zoo, and randomness zoo.
Cantor’s Attic: "Climb into Cantor’s Attic, where you will find infinities large and small. We aim to provide a comprehensive resource of information about all notions of mathematical infinity."
forking and dividing: Map showing classifying various theories according to properties like o-minimality and stability.
pi-base: A community database of topological counterexamples.
Non-well-founded and Cyclic Proof Theory - A Bibliography: This bibliography lists a collection of academic work on the subject of cyclic and non-well-founded proof theory and its applications, consisting of published papers, theses, and preprints.
Kripke Daily Challenge: A Worldle-like where you type modal formulas to find information on a Kripke model which you have to guess.
Hintikka's World: This website shows intelligent artificial agents reasoning about higher-order knowledge (a knows that b knows that...). It enables to explore mental states of the agents by clicking on them. It contains many classical AI examples. It is a tribute to Jaakko Hintikka.
Modal Logic Playground: A graphical semantic calculator for modal propositional logic, built by Ross Kirsling.
Dynamic Epistemic Logic Playground: A graphical semantic calculator for Dynamic Epistemic logic, built by Elliot Evans. Based on the Modal Logic Playground linked above.
LTL Tutor: A free online tutor for Linear Temporal Logic (LTL). Has a blog post explaining it here.
Red Blob Games: "[I]nteractive visual explanations of math and algorithms, using motivating examples from computer games."
Index for "What Is...?": Index for the "What Is...?" column in the Notices of the AMS. This is a collection of short, nontechnical articles aimed at graduate students.
Interesting Esoterica: A list of interesting and unusual papers maintained by Christian Lawson-Perfect.
Using anki for mathematics: Blog with a few ideas for using anki for studying maths. I've tried doing so and had a good experience. It is a ton of work though.
Great Works in Programming Languages: List of important papers in the area of programming languages. Compiled by Benjamin C. Peirce.
Internet Archive Scholar: Internet Archive's scholar research site.
Connected Papers: Tool to visualize (connected) papers as graphs. Good for surveying.
Research Rabbit: Another site for visualising the connection between papers. Never used this much though.
tikzcd-editor: A simple visual editor for creating commutative diagrams.
TEMML: A TeX-to-MathML conversion library.
The Comprehensive LaTeX Symbol List: Over 20000 symbols accessible from LaTeX are listed in a set of tables organized by topic and package.
Unlocking LaTeX Graphics: A YouTube channel teaching TikZ and PGFPLOTS, tools for creating graphics in LaTeX documents. Lots of examples.
TeX by topic, a texnician's reference: A reference manual for TeX.
Natural Number Game (for Lean 4): "In this game, we will build the basic theory of the natural numbers {0,1,2,3,4,...} from scratch. Our first goal is to prove that 2 + 2 = 4. Next we'll prove that x + y = y + x. And at the end we'll see if we can prove Fermat's Last Theorem. We'll do this by solving levels of a computer puzzle game called Lean."
The Principia Rewrite: "We are taking Whitehead and Russell’s original Principia proof on the left, which is really a proof sketch omitting a lot of steps, and filling out the missing steps. For this we are using the interactive theorem prover Coq to ensure each proof step is a valid step according to Principia’s axioms and that no steps are skipped, even by accident. Finally, we are using the Coq encoding to typeset the full, gapless proof as it would appear in Principia’s notation."
The proof is trivial!: Left as an exercise to the reader.

web

MDN Web Docs: References for HTML, CSS, JavaScript. It's from Mozilla, so there is no SEO spam.
QR Code Generator: A QR code generator by Christian Lawson-Perfect
qrcode: Create QR codes using LaTeX.
Squoosh: Image compressing web-app.
Dither it!: Web app for dithering images. Reduces the size of images and gives them an old school look.
Creative Commons: License picker for Creative Commons.
HTML tips: Collection of tips for HTML web pages: semantic HTML, accessibility, etc.
Butterick's practical typography: Free webpage/textbook on typography.
Cheap Bots, Toot Sweet: "This site will help you make a Mastobot! They're easy to make and free (for you) to run."
FediTips: An unofficial non-technical guide to using Mastodon and the Fediverse.
web manifestos: "Netizens are making it known that we are unhappy with the state of the internet as it is today."
ooh.directory: Repository of recently updated blogs.
Wiby: The Wiby search engine is building a web of pages as it was in the earlier days of the internet.
Grow your own service: Site encouraging people to make their own independent online services such as social networks, personal clouds, independent websites and more.
HTML for People: Tutorial for making HTML web pages.
The Plain Text Project: "Plain text. It's boring. It's unadorned. It seems so … well, so archaic. The digital equivalent of papyrus. But it isn't."
Classless CSS: List of classless CSS themes and frameworks. My front page currently uses Simple.css.
The shapes of CSS: Describes how to make various shapes using pure CSS.
Mac Open Web: A collection of open and indie Mac, iOS, and web apps that help promote the open web.
Reasonable Colors: "Reasonable Colors is an open-source color system for building accessible, nice-looking color palettes."
Reasonable.html: "Reasonable.html is a single HTML document that contains a reasonably complete set of example HTML elements."
NeverSSL: "This website is for when you try to open [an website] on a wifi network, and nothing happens. Type "http://neverssl.com" into your browser's url bar, and you'll be able to log on."

entertainment

Standard ebooks: Public domain books as well-formatted ebooks.
Project Gutenberg: Project Gutenberg is an online library of free eBooks.
Game Dad Club: A short introduction site to the Game Dad. "The Game Dad creates Game Time."
Profession: Short story by Isaac Asimov.
What's expected of us: Short story by Ted Chiang.
Vattu: Indie webcomic.
ZQuest Classic: A web version of the classic Zelda engine. Many games available to play.
Antimemetics Division Hub: SCP short story collection about antimemetics: "An antimeme is an idea with self-censoring properties; an idea which, by its intrinsic nature, discourages or prevents people from spreading it."
Architecture of Consoles: "A practical analysis by Rodrigo Copetti"
Foone's death generator: "This creates GAME OVER/YOU DIED screens as seen in Sierra Online's SCI engine, as well as lots of other fake videogame screenshots from many, many games."
FromSoft image macro creator: From Demon Souls to Elden Ring. (Also has a macro creator for Smash Bros. Melee.)
Evangelion title card generator: What it says on the label. Both 4:3 and 16:9 proportions are available.
Fusion tools for Megami Tensei Games: Useful for planning fusions in SMT games. Allows for many methods of search. JSON datasets downloadable in the GitHub repositories.
Awesome Falsehoods: A curated list of falsehoods programmers believe in.
The Cursed Computer Iceberg Meme: "this is not a hall of shame. the intent is to awaken you to many of the peculiarities and weirdness of computers. hopefully, after reading these articles, you will have learned a lot and will embrace chaos."
Draw A Box: "Drawabox is a set of free exercise-based lessons that focus on the fundamentals - the skills you'll need to make sense of all the other resources and tutorials out there."
GENUARY: "GENUARY is an artificially generated month of time where we build code that makes beautiful things."
Tiny Code Christmas: "LoveByte’s Tiny Code Christmas is an event to help you take your first steps in demoscene sizecoding with Lua based fantasy consoles like TIC-80 and PICO-8!"
Six Degrees of Wikipedia: "Enter the titles of two Wikipedia pages in the boxes on this site, click the "Go!" button, and discover just how connected Wikipedia really is."
Iceberger: "Draw an iceberg and see how it will float."

other

Early Modern Texts: Translations of modern philosophy texts into contemporary English.
: "So, what do you do if a topic you want to learn about is not taught at your university? Start your own reading group! And if this sounds like a daunting task, we are here to help. Below, you will find ready-made Blueprints you can use to create your reading group."
The society of mind.: Textbook on AI by Marvin Minsky.
International linguistics olympiads: Like Mathematics Olympiads, but for linguistics.
toki pona tutorial: A free 12 day tutorial for toki pona.
WARNING - Intelligent Malicious Computer Programs: SCP-inspired laptop sticker.
GB Studio: GB Studio is drag and drop game creator for making Game Boy games.
Pixel Art Tutorials: "The biggest collection of pixel art tutorials on the net!"
PGDP - Distributed Proofreaders: Distributed Proofreaders provides a web-based method to ease the conversion of Public Domain books into e-books. By dividing the workload into individual pages, many volunteers can work on a book at the same time, which significantly speeds up the creation process.
Safety Sign Generator: "This is a fancy portal for accessing generators for ERAU Logos, ANSI Z535 compliant Hazard Signs, and Laboratory Door Safety Signs."
Tiny Tools Directory: "This is a list of small, free, or experimental tools that might be useful in building your game / website / interactive project."