File Garden: Where you can store files, and also poorly mimic text.
Burner Kiwi: Temporary email with a custom email address.
Tldraw: Collaborative canvas, also you can draw svg in it! (no login needed).
Telegra.ph: Anonymous "blog" posting, not much else though.
YAY.BOO: A stupid easy place to host stupid simple websites in, for free. File size limit is 10mb.
Webhosts
Neocities: Static webhost with a free tier. I also host my website there!
Nekoweb: Static webhost similar to neocities, with the difference that: no file type limits (for free), better file editor, ability to use NekoVM to build your site with any Javascript SSG or framework, FTP support, and no bandwidth limit!
Leprd: Free web hosting for hobbyists! Seems like a genuine free webhost, though right now registration is closed.
Marigold Town: Free web hosting for hobbyist, personal, artistic, and/or otherwise non-commercial websites with the theme of that every website is a plot of land in a town.
Haza: Free(?) Dobrado (CMS) webhost with support for many IndieWeb protocols! Nevertheless it looks empty (not trying to insult the webhost, it just what it feels like to me).
Lexi's Hosting: Another free webhost, though it has PHP support, and free webmail.
Buruma Hosting: A fairly obscure webhost that if you want to register, you have to email them. Currently open as of writing this.
Colorful-Life: A game of life implementation in full color, fun to mess around with.
DroneWorld: I really don't know what is this, but you can fly around I guess.
webXash: An improved version of Xash3D-empscripten, you can play Half-Life on the browser using this site!
OpenLara: Open-source engine of classic Tomb Raider Series, it's crossplatform but you can also play it in the browser.
Katawa Shoujo: Re-Engineered: A modernization of the classic visual novel Katawa Shoujo, with accessibility features that you can play even in the browser!
Baba Is Wons: Someone remade the majority of Baba Is You levels in their web version of the game, check it out!
Game Engines
Bitsy: A little browser-based engine for little games, worlds, and stories.
Bipsi: A tool to small lo-fi games to play in the browser. Inspired by and an imitation of the Bitsy game maker.