Look at Rackspace's Cloud Files.
They have a complete API, and they are using Limelight. They use containers so you can organize saved data into logical buckets (not directories, as they are not hierarchical). In order to create hierarchical relationships you can either manage it in your own application that touches the API (making your own CDN in effect), and/or name your containers with underscores or other identifiers that you can parse to determine hierarchical relationship.
There's no minimum to using Cloud Files, and I'm pretty sure you can use them with or without a cloud server package.