CUPS
The Linux printing system, which I mostly route around.
CUPS is how printing works on Linux: a daemon, a pile of drivers, and a web interface on port 631 that has looked the same for twenty years. When it works you never think about it. When it doesn’t, you lose an afternoon to drivers for a device that was never really meant to be a printer.
The Brother PTD600 is that kind of device: a handheld label maker with a USB port on it. Rather than hunt for a CUPS driver, I use ptouch-print from the command line, which talks to the thing directly and does the one job I want from it.