Sommer is a very simple library for mapping Plain Old Java Objects (POJOs) to RDF graphs and back. Sommer stands for "Semantic Object (Metadata) MappeR".
Aperture is a Java framework for extracting and querying full-text content and metadata from various information systems (e.g. file systems, web sites, mail boxes) and the file formats (e.g. documents, images) occurring in these systems.
Kowari is an Open Source, massively scalable, transaction-safe, purpose-built database for the storage, retrieval and analysis of metadata. Kowari is written in Java. Kowari supports RDF and OWL metadata.