Favourite, not sure. Maybe my "favourite" would be the one which would be the hardest to replace with something I like.

There wouldn't be something i can think off that could be irreplaceable. However the hardest thing I like may be FanControl.

For the browser, Firefox is very nice, but it's "just" a browser if you think about it. There is brave, and other open source chromium alternatives if it disappears.

For mail clients, I also like the Mailspring design, however Thunderbird just got a new skin and damn it looks good too.

And for the rest, I don't really know. Either I don't remember right now, or no special "like" for the software. Or I like the closed source software convenience more (I may also have no idea of an open source alternative, or an equivalent in features open source).

It depends on the usage really.

Would probably say Firefox, but since many others have already mentioned it, I'll go with Nushell

I have used a lot of stuff over the years but my favorite would have to be a little command line program called cowsay. It takes whatever text you feed it and puts it in a speech bubble above a cow, hence the name.

I’ve been using Logseq after trying Notion and Obsidian a good bit and I’m really enjoying it. It’s a block-based note app that makes connecting thoughts together super easy. So far so goo!

Linux, of course. But another one that I use all the time, and love to death, is SageMath. It's the perfect blend of mathematics and programming for me.

Hard to answer but maybe Haiku or GNU Emacs

Neovim. It's an awesome editor and it has a great community and ecosystem.

https://www.kicad.org/

VS Code

Definitely OpenFOAM. It competes with commercial software that costs thousands of dollars.

ShareX and it isn't even close

QGIS and OpenStreetMap for mapping

GCC, back in the days DJGPP in particular. As a child in the 1990s I could not afford the big name compilers like Watcom. And compared to DJGPP, all the “prized” Borland/Turbo stuff that my middle school prized (with segmented real mode), were practically Fisher-Price and Mattel compilers.

Media Player Classic (I'm unsure if the latest iterations are or even if the Home Cinema edition is open source), TOR, qbittorrent, firefox, thinderbird, obs to name a few that I use regularly.

ReVanced. I love my ad-free, sponsor-blocking, Shorts-removing YouTube experience.

As a bonus, I also enjoy using Mp3tag. It's a program I can use to easily change and update the tags on all my music files, and it can even do it all in batches. It can also connect to various music services (Discogs, Musicbrainz, etc.) to get music tag info directly so you don't have to type it all in manually.

Linux, Firefox, Apache

Voyager, Firefox, Tachiyomi (J2K specifically), Bitwarden, Jellyfin and Findroid, Sonarr, LunaSea...there's so much I can't pick.

Qemu/kvm

Right now, it's Warpinator. Makes at-home wireless file transfers so damn SIMPLE.