Whats your favorite free open source software that everyone should try?

Really going to plug KeePassXC. I think there are several forks for different platforms/slightly different implementations of the KeePass family of password managers, but I prefer the "app that creates a file" paradigm of KeePass to Bitwarden's "server that hosts a database" paradigm.

RedNotebook. It's not 100% what I was looking for in journal software, but it's the closest I tried. For the longest time I kept a journal in plaintext using basically any text editor that fell to hand, but RedNotebook lets me use some formatting and rich text (apparently via YAML or similar markup notation?) and adding pictures/links etc. I do sometimes use my journal to kind of stream-of-conscious-brainstorm, and checklist functionality would be handy for that but any app I've found that provides that is also incomprehensible. I also like that RedNotebook respects my system theme.

AutoKey. You're aware of AutoHotKey for Windows? Well AutoKey runs on Linux, and it uses Python for its scripting language instead of its own proprietary weirdness. I use it all the time.

Gonna mention FreeCAD. FreeCAD probably has the worst case of FOSS disease I'm aware of; it's UI is a klunky mess, it's perpetually unfinished, but if you can survive the utter pain in the ass it is to live with it's extremely powerful. Just the fact that it's a CAD program with a built-in spreadsheet is a total game changer. There's a lot to dislike here, but I honestly don't know what I'd do without it.

Firefox. Everyone reading this already knows everything I'm going to say.

Thonny. A pretty basic Python editor/IDE aimed at beginners and students, but I'm quite fond of it, especially when playing with Micropython on various little microcontrollers.