digiKam
digiKam is a free and open-source digital photo management application for Linux, Windows, and macOS. It provides tools for importing, organizing, editing, and sharing large collections of photographs, with features like tagging, rating, face detection, and geotagging. It supports a wide range of image formats and integrates with online services for photo sharing.
Developers should learn digiKam when working on projects involving photo management, digital asset management, or image processing applications, as it offers a robust example of open-source software in this domain. It's particularly useful for handling large photo libraries, automating workflows with batch processing, and integrating with other tools via its plugin architecture and APIs.