Please write a single answer per comment to clearly see the most popular tools. Vote on the ones you like.
There were too many options in this post and I want to see what are the really interesting ones.
To complement this post:
What file-sharing and media organizing software do you wish that existed?
Audiobookshelf for a self-hosted audiobook and podcast server.
mldonkey cross-platform multi-network p2p daemon
beets music library management and tagging for geeks
Musicbrainz Picard for fetching music metadata and standardizing file naming schemes.
digikam for image and video collection management and viewing (also does duplicate detection)
Deluge run headless. Great Android mobile app.
(didn’t see you posted this one as I was posting it)
yt-dlp youtube-dl fork that is more current and works better for tons of sites to download media.
rtorrent great to download torrents especially on a server.
Spotiflyer downloads playlists from Spotify and many other services.
Plex. I know it’s not the open source option, but I love it.
With plex pass it detects intros and let’s you skip them. This silly feature got me to buy a lifetime pass.
It really surprised me the first time I saw the Skip intro button on a series I downloaded. It seems to work by analyzing the audio and comparing it between episodes from the same season.
Media Center Master the free version is a cool utility for organizing movies/TV shows that will find and attach metadata. The premium license unlocks torrent/Usenet integration and will automatically find download new episodes or movies as they come out based on your preferences
mediainfo Know everything about your medias.
aria2 for downloading files.
mkvtoolnix create/edit mkv files. Very useful to remove or add subtitles or audio tracks amongst others.