[BetaCygni]

Attic + s3cmd sync works fine for me. At least, well enough that I won't be using a closed-source solution. If this is released as open source, great!

[eis]

Unfortunately Attic seems to be abandoned (last commit 10 months ago). But there is a fork called Borg Backup that is actively maintained.

[mahyarm]

With s3cmd, don't you have to have a local copy of the backup somewhere although? So you have the content and the backup at the same spot, you can't directly backup to a remote server that way.

[BetaCygni]

Yes, you need to have a local backup. I actually prefer that, it's another backup location for when files are accidentally (or deliberately) deleted or damaged.