I do still use Calibre on Linux, but it's a good question. There doesn't seem to be a strong alternative for ebook management, and it does have good plugins for DRM removal, etc.
Free and Open Source Software
If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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I recommend https://alternativeto.net/ since you just type in the software name and it will show alternatives rated by popularity. You can filter by many things including "open source".
Firefox as a web browser.
I keep trying email clients, but end up going back to Thunderbird. It still looks clunky, but it works well, and the new UI is in beta, so it should look better soon!
Kdenlive works, and is the best foss video editor imo, but I still haven't found anything as good as DaVinci Resolve (sadly not foss). DaVinci doesn't support common video codecs on Linux though (so you gotta get used to ffmpeg), and for basic video editing Kdenlive works fine, but for making cinematic things like short films or whatever there's sadly nothing nicer than DaVinci. I'd like to be proven wrong though.
I've replaced Windows totally with NixOS(Using the ZFS filesystem as well so I can take /home snapshots and backup easier!). Been a long time Linux user, starting with OpenSuSE.
Anyways, when I did use Windows, my most often used software was LibreOffice for school, VLC for movies, QBittorrent for, well, torrenting, Thunderbird for email and Firefox for browsing(With lots of extensions). I also used Emacs a lot, and still do, here and there.
Overall, I don't really need to use much proprietary software, except for games, of course.
I think Microsoft tried creating something like it on Windows, so I guess it can technically count as a replacement; KDE Connect
For 3D/2D -> Blender
Office stuff: LibreOffice
For programming -> Neovim, Insomnia (for testing out REST api's and whatnot)
Virtual Machines -> KVM/Qemu (Virtmanager, Boxes, etc) This one was a huge improvement for me
I use VLC for most of my multimedia needs
For game development (related somewhat to 3D/2D) -> Bevy and Godot
Plus a tonne of others I'm leaving out. It's really a nice feeling.
Is Insomnia like Postman? If so, could you post a link please?