@pry
oshu: lightweight osu! clone in C++
jq: JSON CLI parser
mtr: traceroute meets ping
perl-rename: renaming files with Perl regexes
nmap: network scanner, very powerful
unarchiver: extracts a huge lot of formats
mat2: trash your metadata!
shellcheck: verify that your shell scripts are nicely done (might need to be took with a grain of salt tho)
strace: must have, logs the syscalls
streamlink: Youtube-dl but for streaming
gpm: get a cursor in your TTY