Command Line Rulez
This is not by all means an exhaustive list, just some of my favorites, and when a new one pops in my head I will add them to the list.
1. lsblk: list block devices, useful to see what is attached to the computer.
2. dd: byte-by-byte copy of disk images and dealing with them (pack/unpack).
3. ranger, spf: probably the best command line file managers with Miller columns.
4. lfm, vifm, mc: dual-pane file managers.
5. btop: a bit fancier than htop, for viewing running processes and more.
6. irssi: command line IRC client, simple and powerful
7. fastfetch, neofetch: quick system summary
8. sudo hdparm -t --direct /dev/sd? to check the speed of a drive
9. fzf fuzzy finder, search tool
10. asciinema to record terminal sessions and share them later
11. ffmpeg to deal with audio-video files and convert them
12. lz4 fast lossless file compression
13. mpv is a command line media player for audio and video
14. curl cheat.sh the best-last cheat sheet you will ever need!
15. tlp to manage your laptop battery
16. ddrescue (install gddrescue) useful data recovery tool
17. ddgr to search DuckDuckGo from the terminal
18. hexyl is a simple color hex viewer for the terminal
19. toot is mastodon for your terminal
20. gifsicle to create and manipulate GIF files
21. abcde to grab and compress a CD to ogg/mp3/flac whatever
22. exiftool for meta information of files, e.g. pictures
23. chafa to turn graphical GIFs into ANSI character based animations
24. history to list the last max 1000 commands every typed and choose from the list
25. ctrl-r in the command line to search in the history of the last 1000 commands
26. aerc for email in the terminal
27. ncdu for checking disk usage
So much for now. Will add more if I remember... Peace :)