Open-Source Tools

Music Downloaders:

Chimera (Deezer, Tidal, Qobuz)- https://notabug.org/Aesir/chimera.
Deezloader Remix (Discontinued project but still working)- https://notabug.org/RemixDevs/DeezloaderRemix.
Deemix (from the Deezloader devs)- https://notabug.org/RemixDev/deemix. This is still in the works.
Soundcloud-dl- https://github.com/jbkuczma/soundcloud-dl.
Spotify-dl- https://github.com/SwapnilSoni1999/spotify-dl
Spotify-dl alternative- https://github.com/ritiek/spotify-downloader.
Savvn Downloader- https://github.com/monuyadav016/Saavn-Downloader.
**Note, the spotify downloaders do not breaks Spotify's DRM, but download from youtube and apply the right metadata.**

Anime and Manga Downloaders:

Manga:

Manga-dl- https://github.com/manga-py/manga-py.
Manga/Comic downloader- https://github.com/Xonshiz/comic-dl.
Manga/Anime Downloader- https://github.com/manga-download/hakuneko.

Anime:

Monkey-dl: https://github.com/Oshan96/monkey-dl.\n
Crunchyroll ripper- https://github.com/s32x/anirip.
Another crunchyroll ripper- https://github.com/ThePBone/CrunchyrollDownloader.
A third crunchyroll ripper- https://github.com/hama3254/Crunchyroll-Downloader-v3.0.
A fourth, final crunchyroll ripper- https://github.com/DasKraken/CR-dl.
Anime downloader- https://github.com/vn-ki/anime-downloader.
Anime Android Streaming App- https://github.com/mukul500/AnimeXStream. Features: P-I-P mode, no ads, epic.
For more tools check out this: https://github.com/topics/anime-downloader.

Other Useful Tools:

Subtitle Downloader- https://github.com/manojmj92/subtitle-downloader.
inAudible Audible DRM-Ripper- https://github.com/rmcrackan/inAudible. Note that, development was not on Github but the installer is.
Pearson eText to PDF Downloader- https://github.com/NoMod-Programming/PearsonEbookDownloader.

Video Downloaders:

bmovies.is downloader- https://github.com/vaulstein/fmovies.
fmovies.to downloader- https://github.com/shb9019/FmoviesDownloader.
youtube-dl- https://github.com/ytdl-org/youtube-dl. Arguably the best tool for downloading youtube videos.
Annie Downloader- https://github.com/iawia002/annie.
Udemy Downloader- https://github.com/FaisalUmair/udemy-downloader-gui.
Udemy-dl- https://github.com/r0oth3x49/udemy-dl.

XTREME Download Manager- https://subhra74.github.io/xdm/.