Duplicacy is an interesting free-for-personal-use backup tool with a host of professional features.

There's support for backing up to Dropbox, Amazon S3, Backblaze, Microsoft Azure and Google Cloud Storage, as well as any SFTP server or your own local disk.

Backups are optimised for extreme efficiency. File and block-level deduplication means Duplicacy doesn't waste time storing multiple copies of the same data, and the incremental backup scheme ensures the program only backs up what's changed.

Simplicity is also important. You don't have to understand or even care that backups are incremental, because they all behave as though they're full images for restoring data, and you can delete any backup without trashing the others.

The program also supports multiple clients backing up to the same storage space, and the deduplication feature can make this a very good idea (different clients get to share identical chunks of files automatically).

There's a very simple scheduler, some snapshot management tools, and an option to send email notifications after scheduled backups.