Various stuff I use and I’m happy with:
Software
Operating system : GNU/Linux (mostly Ubuntu Server, Debian and Gentoo)
Text editor : GNU Emacs
Window manager : XMonad, EXWM (Emacs X Window Manager)
File manager : dired, ranger, rox
Terminal emulator : urxvt
IRC client : Weechat, Circe via Weechat Relay
Web browser : Mozilla Firefox, surf + tabbed
Email : notmuch
Lockscreen : slock
Slideshows : sent, Reveal.JS, Beamer
Calendar : TrayCalendar
Calculator : Emacs Calc
Version control system : Git, RCS
Backups : Borg
Data analysis : R