Linux

losetup

it's useful for dealing with virtual disk images. like a real physical hard disk, but it's a file on the computer. you can mount it, format it, and write it to a real physical disk.

it's sometimes used with virtual machines, with iso images, or when preparing a bootable disk.

motion

After spending years dealing with shady freeware and junk software on windows, I was floored by how easy and nonchalantly I was able to set up a simple security camera on my PC

ip eg:

# ip a
# ip a a 192.168.1.99/24 dev enp160

The first incantation - ip address (you can abbreviate whilst it is unambiguous) gets you a quick report of interfaces, MAC, IPs and so on. The second command assigns another IP address to an interface. Handy for setting up devices which don't do DHCP out of the box or already have an IP and need a good talking to.

Oh and you can completely set up your IP stack, interfaces and routing etc with it. Throw in nft or iptables (old school these days - sigh!) for filtering and other network packet mangling shenanigans.

tmsu is pretty cool - it creates a little db and uses that to track tags on your files without ever touching them. It also has it's own little tag based filesystem.

A really simple one but surprisingly useful is cal

all of them

Find

GNU datamash (https://www.gnu.org/software/datamash/alternatives/) - handy tool for data munching. There's also https://github.com/BurntSushi/xsv

Lightweight sudo alternatives, hard to google too. I know ssu and rdo, please mention others.

paste. I don't think a lot of people know this command, but it can be handy at times