S3cmd

Command Line S3 Client and Backup for Linux and Mac

Screenshots

Hunter's comment

Sync or backup your servers using this little tool is a simple task and it is tiny. Using command-line tools for daily chores or scripting is a necessity for anyone (like me :-) ) working with servers, and while there are large dedicated packages that do the same job as this little tool, why waste the resources?
I'm using it mostly with DigitalOcean spaces wich are S3 compatible. https://www.digitalocean.com/docs/spaces/resources/s3cmd-usage/

"S3cmd is a free command line tool and client for uploading, retrieving and managing data in Amazon S3 and other cloud storage service providers that use the S3 protocol, such as Google Cloud Storage or DreamHost DreamObjects. It is best suited for power users who are familiar with command line programs. It is also ideal for batch scripts and automated backup to S3, triggered from cron, etc."

There is a Windows version aswell!
S3Express