People have mentioned SyncThing and Seafile as straight backup options.

Another option one could consider is FolderSync[1]. It has a number of cloud back-ends it can interface with, but I use it to periodically SFTP to my home server to backup several areas in my phone, including photos, camera and app backups.

Two niceties for me it has:

* Two-way sync. I'll often clean up my camera roll on my PC, and that syncs back to the phone.

* Only attempts to connect to me home server if it is on my home wi-fi.

[1] https://play.google.com/store/apps/details?id=dk.tacit.andro...