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?
deleted by creator
digikam for image and video collection management and viewing (also does duplicate detection)
beets music library management and tagging for geeks
Musicbrainz Picard for fetching music metadata and standardizing file naming schemes.
mldonkey cross-platform multi-network p2p daemon
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
rtorrent great to download torrents especially on a server.
Deluge run headless. Great Android mobile app.
Spotiflyer downloads playlists from Spotify and many other services.
mediainfo Know everything about your medias.
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.
(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.
aria2 for downloading files.
Sonarr, Radarr, qBitorrent, SabNzbd, Jackett and frontends like Kodi and Jellyfin. Jackett can interrogate a whole bunch of public and private torrent trackers for you, fetch a .torrent file given a set of criteria (like are there seeders) and put it in your torrent client for you. It does all this automatically from Sonarr/Radarr once set up in them.
I set all this up in my server using my phone and an Android SSH client from my bed while recovering from nasty painful surgery and it’s been pretty solid once I worked out the kinks since.