WordPress plugin to store uploads on S3. S3-Uploads aims to be a lightweight "drop-in" for storing uploads on Amazon S3 instead of the local filesystem.
It's focused on providing a highly robust S3 interface with no "bells and whistles", WP-Admin UI or much otherwise. It comes with some helpful WP-CLI commands for generating IAM users, listing files on S3 and Migrating your existing library to S3.