Borg for backup. I'm really surprised it's not more widely known. It's an incredible piece of software.
Also, not really lesser known software, but a lesser known feature of file systems including the ones we use in FOSS operating systems: extended file attributes - useful to add metadata to files without modifying them.
As an add-on (sort of) to Borg, I was told about Vorta yesterday and installed it to run scheduled, encrypted backups of my local machine to an external drive, but you can also ssh to a remote server if you wish. Works like a dream.
There's also Rustic. It uses the same repository format as restic. It already has some pretty neat features and since latest release a ton of built-in backends.