I replaced all my Dropbox uses with SyncThing (and love it). I run an instance on my server at all times and on every client.

+1 for SyncThing

I have it installed on my immediate family's devices to ensure all the photos are auto-backed-up to our NAS (which is then backed up to another NAS).

I need to check to make sure it's still working once in a while (every couple of months), but it's usually fine, and even if it's somehow stopped working, getting it running again catches itself up to where it should have been anyway.