Launcher for mod. Minecraft
Prism Launcher

A newer version of the older PolyMC, which has seperated from MultiMC.
Allows the downloading of mods and modpacks from Modrinth and CurseForge.

Server console
Crafty Controller

Switched to it from Minehut after they (partially) pay-walled it.
More exist but this one suits me the most.
We use it in combination with RadminVPN.

Server hosting
Minehut

Switched to it from Aternos after all the lags on there started to get too annoying.

Skin editor and modeller
Blockbench

Not only for Minecraft, which makes it quite the powerful and useful tool.
Some of the web alternatives for skin editing are: Novaskin, SkinMC, Skindex, Planet Minecraft.

Mod, texture, shader downloading
Modrinth

In some aspects better than CurseForge but some things (like mods) may be missing.

OptiFine alternatives
list here

OptiFine never really worked that well, so in today's age I'd recommend to switch to Sodium and the above mentioned alternatives.

Server plugin downloading
SpigotMC

There's no better and more comprehensive site, at least in my opinion.

Maps
Minecraft Maps

Nowadays the best website for downloading maps.

Maps, skins, textures
Planet Minecraft

Classic community for Minecraft where you can find almost anything.

Minecraft Bingo
v1.16.5

A bit silly, but I've always found it entertaining.
Good alternative to speedruning.

Speedrun
here

Speedruning community, i.e. leaderboards.

Cheatsheet with formatting tags
on Minecraft Wiki

Also a great source for all information about Minecraft.

Conv. worlds between ed.
Chunker

Haven't had an opportunity to try it out so far, but it's very recommended by the community.

ViaVersion - Which do I need?
Tool zde

ViaVersion is a server plugin which allows players to play together on one server even with different version of the game (e.g. 1.8 with 1.21).

Server tools
Collection here

Already proved useful to me multiple times while running my small server.

World mass-editing
Axiom

Replaced older MCEdit. For smaller edits, I recommend classic WorldEdit (mod/plugin).

Resource pack
Stay True

I like its style.

Dark menus
Unique Dark

Does what it says on the tin. I like it more, but it's a matter of taste.

Animations
for Mobs | for Players

Really cool, but requires EMF and ETF.

Differentiators
Enchanted books | Potions

Great for reducing confusion in chests but requires CIT Resewn.

Shadery
Complementary

May be more demanding, but for a stronger computer it's breathtaking.