Foliate. It's an ebook reader. Fast, functional and immersive. I like using it in full screen mode, dark mode. Sometimes it feels nicer than a kindle device.<p>What I like about it is, like any excellent software, you don't notice it. It gets out of your way and lets you do your task.
For mature software, Vim.<p>For new software, I'd say 1Password. It's amazing how well it syncs with everything, across Firefox and Chrome, and how it handles not just usernames and passwords but gives a secure way to share secret plain text files, scan QR codes and more.
Taskwarrior [1].<p>Brilliant utility. Surprising amount of well thought out features. Very flexible.<p>The C++ source code seems well organized on first glance.<p>[1] <a href="https://taskwarrior.org/docs/" rel="nofollow">https://taskwarrior.org/docs/</a>