Metadata

Seafile is a self-hosted file syncing and sharing solution that offers flexible pricing plans for teams of all sizes. It provides both a Community Edition, which is free and open-source, and a Professional Edition with advanced features and support.

Open Library is a universal catalog for book metadata, offering free access to book data.

The Flickr API provides access to a vast collection of photos and metadata, allowing developers to create applications that interact with Flickr's photo-sharing platform.