Wallpapers:
r/wallpapers: No explanation necessary
https://reddit.com/r/wallpapers

r/EarthPorn: Has lots of stunning high-res pictures of nature
https://www.reddit.com/r/EarthPorn/

WallpaperHub: Has many official Microsoft wallpapers
https://wallpaperhub.app/

Microsoft Design Wallpapers: Even more official Microsoft wallpapers
https://wallpapers.microsoft.design/

Unsplash: A site with high-quality royalty-free images and wallpapers
https://unsplash.com/

Wallpaper collection by <@645751493450006548>:
https://actoweb-my.sharepoint.com/:f:/g/personal/avogato_catphile_actoweb_xyz/EimxR6YeSqpOsZUYKD77I-IBSAZfLNAASmSYx67rTF5CuQ?e=hfSubn

Lively Wallpaper: An app for animated wallpapers
https://livelywallpaper.net/

Wallpaper Engine: A paid app for animated wallpapers
https://www.wallpaperengine.io/

WinDynamicDesktop: macOS-style dynamic wallpapers on Windows.
https://github.com/t1m0thyj/WinDynamicDesktop

Rainmeter: An app to add rich widgets (called 'skins') to your desktop
https://www.rainmeter.net/

SecureUxTheme: An app for non-destructive theme patching for theming win32 apps
https://github.com/namazso/SecureUxTheme
​
Shell customization:
TranslucentTB: An app to change the blur behind the taskbar
https://github.com/TranslucentTB/TranslucentTB/

DwmBlurGlass: Adds Aero-like transluceny to the system titlebar on Windows 10 and 11
https://github.com/Maplespe/DWMBlurGlass

StartAllBack: An app that replaces the taskbar and Start menu adding a variety of features
https://www.startallback.com/

MyDockFinder: A paid macOS-like dock and menu bar for Windows
https://store.steampowered.com/app/1787090/MyDockFinder/

komorebi: A tiling window manager for Windows
https://github.com/LGUG2Z/komorebi/

Yasb: A Unix-like status bar for Windows
Makes a great combination with komorebi
https://github.com/denBot/yasb

Stardock: A company that makes a huge variety of paid shell customization utilities
https://www.stardock.com/products/

Terminal/Shell customization:
Windows Terminal: The modern first-party terminal experience for Windows with profiles, themes and more
https://learn.microsoft.com/en-us/windows/terminal/

Windows Terminal themes: A large collection of Windows Terminal colour schemes
https://windowsterminalthemes.dev/

terminal.sexy: A terminal colour scheme editor
Does not natively support Windows Terminal but can be used through https://heftymouse.github.io/tsconverter/
https://terminal.sexy/

PowerShell 7: The modern cross-platform PowerShell
https://learn.microsoft.com/en-us/powershell/

Oh My Posh: A customizable cross-shell prompt
https://ohmyposh.dev/

Starship: Another customizable cross-shell prompt
https://starship.rs/

Terminal-Icons: A PowerShell module to add file and folder icons to Get-ChildItem and other cmdlets
https://github.com/devblackops/Terminal-Icons

Winfetch: A neofetch-like system information utility for Windows written in PowerShell
https://github.com/lptstr/winfetch

Miscellaneous:
PowerToys: Has a variety of useful utilities including customization programs like FancyZones
https://github.com/microsoft/PowerToys

ShareX: An advanced screenshot and screen recording program
https://getsharex.com/

AeroShot CRE: An app that takes transparent screenshots of individual windows with drop shadow
https://apps.microsoft.com/store/detail/9NNBCDGN1ZS8

Auto Dark Mode: Automatically change theme and other things based on time of day
https://apps.microsoft.com/store/detail/auto-dark-mode/XP8JK4HZBVF435

AccentColorizer: Brings your Windows accent colour to more places
https://github.com/krlvm/AccentColorizer

Mica For Everyone: Allows customizing the material and theme of default titlebars
https://github.com/MicaForEveryone/MicaForEveryone