OpenSSH, and a few years ago OpenSSL (look where that got us).
At higher level:
Calibre (everything ebook related)
Inkscape (vectorial drawing tool)
Markdown/LaTeX/RestructuredText
Bash, after a gazillion years still defacto shell, I've tried Zsh and Fish but they are :slower and just add things I don't care about (in facto my current Bash prompt looks better than any available theme for those) and when you get used to them you ssh to some standard machine and all the muscle memory goest to hell.
All RSS/Atom related: simple, fast, non-traceable, clean.
Finally not SW per-se but a PL; C, people talk about it like a dead language even tho maybe 80% of their daily SW is written on it, your OS, Webserver, Browser, if it runs fast is probably C, I don't include C++ here because some love goes to it but still very under-appreciated. just a few years ago, with Rust we got some potential replacement for C++, we are not even sure if it'll come as performant as plain C, so we have C for maybe decades ahead and still looked down as a dead language.
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OpenSSH, and a few years ago OpenSSL (look where that got us).
At higher level:
Finally not SW per-se but a PL; C, people talk about it like a dead language even tho maybe 80% of their daily SW is written on it, your OS, Webserver, Browser, if it runs fast is probably C, I don't include C++ here because some love goes to it but still very under-appreciated. just a few years ago, with Rust we got some potential replacement for C++, we are not even sure if it'll come as performant as plain C, so we have C for maybe decades ahead and still looked down as a dead language.