The sort of program that once set up, just ticks along without fuss or bother forever.
For me, as I’m replacing the vms today which I set up five years ago and haven’t needed to touch since;
- HAProxy
- KeepaliveD
Not easy to learn, but once they’re running, they both go on forever.
Probably something that is so basic or integrated into the distros I use that I forget about it because every time I use it, it just works.