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
Musicbrainz Picard for fetching music metadata and standardizing file naming schemes.
beets music library management and tagging for geeks
Jellyfin for a free and open-source media server and suite of multimedia applications.
yt-dlp for downloading videos from various websites.
This one is amazing, especially when you realise various websites means, nearly any streaming platform. SoundCloud, Mixcloud, Bandcamp etc, it knows them and can download whole archives of files. As we watch Twitter and Reddit totter and crumble it’s time to realise this will happen with the sites that host your favourite podcast, unless you make offline copies
I use this one. Extremely fast and easy to use once you figure it out.
Recently found out about Stacher which is a GUI for yt-dlp
Calibre-Web for a web app providing a clean interface for browsing, reading, and downloading eBooks using an existing Calibre database.
Calibre for organizing, converting, and syncing eBooks.
gallery-dl for a command-line program to download image galleries and collections from several image hosting sites.
RetroArch for a frontend for emulators, game engines, and media players with support for various platforms.
Nicotine+ for a graphical client for the Soulseek peer-to-peer network.
Sonarr for a PVR for Usenet and BitTorrent users that can monitor multiple RSS feeds for new episodes of your favorite shows.
ffmpeg the Swiss army knife for everything video and audio.
mldonkey cross-platform multi-network p2p daemon
digikam for image and video collection management and viewing (also does duplicate detection)
