Websites

DoubleDouble - Music Downloader for Spotify, Soundcloud, Qobuz, Deezer and Tidal.

YtmDL Web - Best looking and performing online YouTube music downloader.

RuTracker - Russian tracker with a lot of Unknown or Foreign Music.
Requires Sign-up.

SoftArchive - SoftArchive's Music Section, has many different Albums you might not find elsewhere.

KHInsider - Offers plenty of Video Game Soundtracks and Game audio rips. But audio quality tends to be extremely low.

Sitting on Clouds | Hikarinoakariost | Sukidesuost | Nyaa - Very good and reliable sources to get mainly Anisons from, but also other Asian Music as well. Sitting on Clouds also has a good collection of Game OST. They use friendly file hosters except Nyaa, it uses Torrents instead.
AnimeThemes | Themes.moe - A huge collection of Ending/Opening Themes for Anime. Right click to download the Audio/Video.

Squidify - A Web Player for Game OSTs/Anisons/Japanese Music.
You can also stream locally, more info at https://wotaku.moe/guides/squidify/

Unreleased Disco - Huge archive of Unreleased Hip-Hop/Pop Songs from various Artists.

Tools

SoulseekQT - P2P Music Sharing Client, can have some niche and hard to find Music.
        Nicotine+ - A graphical client for Soulseek that is frequently updated unlike Soulseek and has added features.

Deemix & d-fi - Deezer downloaders. Deemix can convert Spotify playlists into Deezer playlists to download. d-fi can also accept Spotify or Tidal song links and download them from Deezer. They can download up to 128Kbps quality with a free account.
Deemix is abandoned, if you're having issues use d-fi or streamrip instead.
For higher quality downloads, a paid Deezer account is required.

Qobuz-DL - Qobuz downloader. Supports downloads of last.fm, Spotify and Apple Music playlists through Qobuz.
Requires a paid Qobuz account.

Tidal-Media-Downloader - Tidal downloader in CLI. General guide for installation (guide for Tidal-gui may be outdated).
        Tidal-Media-Downloader-PRO - GUI version.
Both require a paid Tidal account.

SCDL - SoundCloud Music downloader, can set id3tags to downloaded Music too.

streamrip - A Scriptable Music downloader for Qobuz, Tidal, SoundCloud, and Deezer. Supports downloads of Spotify and Apple Music playlists through last.fm.
Requires a paid Qobuz/Tidal/Deezer account.
OrpheusDL - Music downloading client.
All plugins listed below require a paid account in their respective platforms except Musixmatch.
        OrpheusDL-Tidal - Tidal downloader plugin.
        OrpheusDL-Qobuz - Qobuz downloader plugin.
        OrpheusDL-Deezer - Deezer downloader plugin.
        OrpheusDL-SoundCloud - SoundCloud downloader plugin.
        OrpheusDL-Napster - Napster downloader plugin.
        OrpheusDL-Musixmatch - Musixmatch Lyrics downloader plugin.
        OrpheusDL-KKBox - KKBox downloader plugin.

Zotify - A highly customizable Music and podcast downloader for Spotify.
This tool downloads songs directly from Spotify. Use a burner account.
spotDL - Download your Spotify playlists and songs along with album art and metadata fetched from YouTube.
This tool does not download songs directly from Spotify.

SpotX - Block video, audio and banner ads on Spotify, also unlocks some Premium and Experimental features.
        SpotX-Bash - Linux/Mac version.
BlockTheSpot - Video, audio & banner adblock/skip for Spotify.

YT-DLP - Can download from over 1500 sites including: Crunchyroll, Funimation, SoundCloud, Bandcamp, PornHub and more. Link to all supported sites.
Actively maintained fork of the popular YouTube Downloader youtube-dl with more functionality.

dzunlock - A browser UserScript that allows up to Hi-Fi streaming on Deezer.

Communities

Slav Art - Allows downloading tracks from Qobuz.
Their Divolt Server on the other hand, has multiple bots that can download from Qobuz, TIDAL, Deezer, Soundcloud, Spotify, Apple Music, JioSaavn and YouTube Music. They allow community Music requesting/sharing.
Info regarding Divolt and how it compares to Revolt.

SquidBoard - A forum for sharing and requesting any type of music including Game OSTs, Anime OSTs, TV music and more.
Requires Sign-up.

Doujin Café - The source of nearly all doujin rips online.
Discord Server.

r/xTrill - A Subreddit for requesting and sharing Music.

r/MusicHoarder - A SubReddit for general Music enthusiasts, they allow requesting and sharing Music. Check their Wiki for some useful information.

Misc

Firehawk52 Guide - A Mega-Guide for tools and ARLs to rip Music from popular streaming services.
THE MEGA MUSIC RIPPING GUIDE - A Music ripping guide for various platforms by Esoteria Avenue.

StillFreeDeezerARL - They share Deezer ARLs (used for ripping Music with Ripping Tools).

Spek - Cross-platform GUI spectral creation program with easy drag and drop. Beginner-friendly but spectograms are not as detailed/clear as SoX.

SoX - Cross-platform CLI Swiss Army knife of sound processing programs, generates clean high quality spectograms with loads of additional features like collaging multiple spectograms and even applying various effects to these sound files. First time users may be confused by the vast features and are hence recommended to read the documentation once.

hey-bro-check-log - A CLI log checker maintained by Doujin Café Discord. Forked from the official hbcl repository.

CUETools - CUETools is a tool for lossless audio/CUE sheet format conversion. The goal is to make sure the album image is preserved accurately.

Deezer ARL Checker - Deezer ARL checker, useful for verifying ARLs.

Tune My Music - Transfer your music library from any music service to any other you want. Supports Spotify, TIDAL, Apple Music, YouTube, Amazon Music, Pandora and many more.