Tutorials

EJ Media HTML and CSS Crash Course - used this playlist to learn :D The guy is an amazing teacher!
W3Schools - I struggled to learn with it, but I frequently use it for reference :D
A Complete Guide to Flexbox - i always use this as reference
Beginner's Guide to HTML and CSS - Helped me understand the basics :)
Melon's Website making Tutorial - Fun and basics
Fun with HTML and CSS - more tips and tricks, this time from the Yesterweb zine :D
Sadgirl's Tutorials - Tips, tutorials, references and more!
GIF Stamps Tutorial! - how to make animated stamps using Photopea

Widgets and decor

HTML Comment Box - Comment section for your website! Free plan available.
Ayano's Comment Widget - A different option for comments on your website! Supposedly more customizeable and Neocities-friendly.
123Guestbook - a guestbook for your site allows people to leave nice comments :D
MiniGB - Another option for guestbooks/comments - it is currently in alpha.
Chatbox - the chatbox you see on so many people's websites on Neocities.

Webamp - Fully functional Winamp in your website! Very customizeable music player with lots of skins made for it :D
How to Embed WINAMP on Your Website - this has helped me A TON
Solaria's Simple Jukebox - simple music player

Free JavaScript Effects - cool effects for your mouse, such as sparkles above your cursor :D
Pollcode - create and embed polls on your website
FC2 Counter - easy and customizeable visitor counter.

Fancy Parts - graphics like background patterns, dividers and banners
Fool Lovers - literally every graphic your heart desires
Asterism - fonts, graphics, and more!
Whimsical - awesome original graphics
Bitmap Dreams - adorable graphics
King Lulu Deer - awesome pixel artist
Catch a Star - adorable website with adorable graphics
Hamster Cage - Hamtaro adoptables!
Sushi Cat - fortune cats and sushi adoptables
Kawaii Sozai Web Graphics - collection of links to mostly Japanese sites offering free graphics
Capstasher Button Collection - Massive collection of 88x31 buttons
CakeBatter - massive collection of favicons
Blinkies Cafe - blinkie generator!
Bouncy Bubbles - archived website with tons of cute pixels
The Cutie Internet Archive - an archive of cute pixels and websites
Ichigo Directory - directory of cute websites
Kawaii Attic - another directory for cute websites

Plots Modeller for Minecraft - create shapes like spheres, ellipses and towers
Pixel Circle Generator - another shape generator, but only for circles
Minecraft Shapes - even more shape generators, including triangles, hexagons, octagons, etc.
Chunkbase - look for biomes or structures in seed maps
Block Palettes - palettes for your builds
Nether Portal Calculator - easily link up nether portals
Enchantment Ordering Tool - calculate the most efficient enchantment order for your tools and armor
Technical Minecraft Wiki - in-depth explanations of Minecraft mechanics
Discontinued Features Wiki - documentation of many bugs and removed features with instructions on how to recreate them
List of Useful Mods - performance and bugfix mods
Prism Launcher - launcher that I'm primarily using!
BetaCraft Launcher - launcher for old versions of Minecraft, with a repository of mods.
Modrinth - download mods
Modpack Index - search engine for modpacks
Hangar - plugins for PaperMC servers
SpigotMC - plugins for Spigot, work for PaperMC as well
How to set up a Minecraft server hosted on your own PC, for free, using playit.gg instead of port-forwarding
Paper Server Optimization Guide - optimize your PaperMC server
Ianxofour - makes very simple and optimized farms, with step by step tutorials

32bit Cafe Resource List - So many useful resources listed here!
TamaTalk - website dedicated to Tamagotchi!
Spriters Resource - lots of sprites from different games
/r/Worldbuilding Resources Master List - massive document with resources that help with worldbuilding!
How to run a small social network site for your friends - an article