These are some lightly categorised sites or pages I think are worth reading. For the most part, these sites contain a lot of content, so if you want links to more fine-grained and specific things that I find interesting then you should look at my blog, where I basically just post links to stuff I find that doesn't belong on this page.
https://abstractmath.org/ has some maths stuff on it.
Complex Projective 4-Space
"Where exciting things happen", https://cp4space.hatsya.com/ hosts a blog about "interesting aspects of mathematics" covering a wide variety of topics, sometimes with some code thrown in for good measure.
Linking to a youtube channel
doesn't really fit the theme of this page, but this one has enough interesting
stuff and is obscure enough that I'll make an exception. It contains more than
600 700 (and rising) interviews with "top intellectuals and
academics". There's enough stuff that something will probably interest you.
EuSpRIG Horror Stories
The European Spreadsheet Risks Interest Group, which does what it says on the tin, maintains a list of some of the spreadsheet failures impactful or interesting enough to be reported in the global news. You can read it at http://eusprig.org/horror-stories.htm.
https://erich-friedman.github.io/ is (unsurprisingly) Erich Friedman's site. Contains lots of fun stuff, including distinctive facts about some (many) numbers up to 9999, this paper on square packing, and enough puzzles to keep you going for a while.
E. W. Dijkstra Archive
https://www.cs.utexas.edu/~EWD/ — an archive of transcriptions of most of Edsger Dijkstra's 1318 "EWD" manuscripts, as well as a few other bits. It's nice to see important history preserved in a usable format, and at least now I can't complain I have nothing to read. Most of the actual content is fairly accessible too.
The best and most comprehensive analysis of the matrix films and surrounding media. https://matrixresolutions.com/. Contains interesting theories, debunks lesser interpretations, and has line-by-line analysis of significant scenes.
Playing to Win — Becoming the Champion
By David Sirlin, available at https://www.sirlin.net/ptw/.
This is a short book primarily about competitive games. It's insightful, realistic, and truthful, and worth reading even if you don't play any of those kinds of games.
Pluralistic: Daily links from Cory Doctorow
https://pluralistic.net/ is Cory Doctorow's blog, with articles about digital rights, economics, and politics. He posts almost daily, and since he's been blogging for so long so consistently, has unlocked the ability to include lots of things he himself wrote in his "This day in history" section every time. "Optimized for Netscape Navigator."
https://ridiculousfish.com/blog/ has some cool stuff on it, particularly this article about where the magic number comes from when division by a constant is replaced by multiplication and bitshifts (and why that works in the first place).
Search the Internet
https://search.maginalia.nu hosts a search engine where you can find new things, rather than whatever the obvious names are showing these days.
sndkit — a sonic toolkit for computer music composition
https://pbat.ch/sndkit/. "It aims to bring a collection of useful audio DSP algorithms together in one ecosystem, and document how they work."
The finest journal in the field of satirical linguistics, at https://specgram.com/. Released quarterly (though this is changes about as often). Inimitable and speaks for itself.
https://www.surfaces.cx/ — an "open literary arts platform [...] working to cultivate new literary terrain [...] exploring the textures of dislocation, technology burn-out, and pain".
The Xenharmonic Wiki
A collection of resources and so on for microtonal/xenharmonic music, which you can find at https://en.xen.wiki/. It's a good place to learn about the topic.