The main one I have is Syncthing[0]. I have a fairly involved file sync setup across several Android and Linux devices and it works beautifully; I like that I can use the space available on my devices instead of having to depend on/pay money for cloud (I know that it is not much for most of us). However, to best of my knowledge, Syncthing doesn't run on iPhone (it is possible that jailbreaking etc can make that work but that is a whole different area).

[0]: https://syncthing.net/