Favourite software that nobody talks about: your hidden gems

[FrostBear]

Not the obvious answers. Not the software that comes up in every productivity thread. The things you use regularly that you almost never see mentioned, the tools that solve a specific problem so well that you can barely imagine how you managed without them.

Could be free and open source, could be paid, could be something obscure that does one very specific thing perfectly. The criterion is that it is genuinely underappreciated relative to how useful it is.

I will start when I think of the clearest example from my own workflow but I want to hear yours first.

[DodgyCoder]

Syncthing. Peer to peer file sync without any cloud involvement. Runs in the background, always works, never loses anything. I have used it for four years and it has never given me a reason to think about it, which is exactly what you want from infrastructure software.