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".
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 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.
I think Microsoft tried creating something like it on Windows, so I guess it can technically count as a replacement; KDE Connect
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.
Notepad->kate
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.
One of my favorite is fsearch, basically a one-to-one copy of Everything that is written in GTK
I use Claws Mail for my mail stuff. It works really well with my self-hosted email server and I really like the UI and such. On Windows I used to use the mail client that was pre-installed.
For text editing I've switched to vim and emacs (doom). I usually use vim for quick little edits to things that aren't worth opening up emacs and finding it in there and I use emacs for anything that'll take some time. On Windows I used VS Code.
For a web browser I use Firefox, hardened with arkenfox/userjs. Never had any problems with it and have been using it since before switching to Linux.
I find LibreOffice to be way too heavy for light notes (My current light text editor is FeatherPad), but it's a great alternative to Word!
There is also abiword for a relatively lighter weight word processor that handles doc files. Although I also lean to using a text editor most of the time. Right now my favorite is kate.
I guess...
Telegram -> Telegram FOSS
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