eXist-db
eXist-db is an open-source NoSQL database management system built entirely around XML technologies. It stores XML data natively, provides XQuery and XPath for querying, and includes a built-in application server for web-based applications. It is designed for document-centric applications, digital humanities, and archival projects where XML is the primary data format.
Developers should learn eXist-db when working with XML-heavy applications, such as digital libraries, content management systems, or scholarly editions, as it offers efficient storage and querying without requiring XML-to-relational mapping. It is particularly useful in academia, publishing, and cultural heritage sectors where standards like TEI (Text Encoding Initiative) are common, providing robust support for complex XML structures and full-text search.