So maybe you guys all already know about Syncthing, but I had never heard about it before.
Syncthing in essence does something very similar to Dropbox, iCloud, or OneDrive, but it does so directly between your various devices (no iOS yet though) without involving any 3rd parties and their clouds where you’re effectively handing your data over to somebody else. Syncthing is open source, free, and it supports macOS, Win, Linux, and Android. Syncing is configured through a very simple web interface, the actual syncs are encrypted of course. Versioning, access control/permissions, and extra options like send/receive only are included.
Leo Laporte did a brief but very interesting segment about it on his show this week (that’s how I learned about it).