Zemantic is an RDF module for Zope (read its announcement). From what I read (not tested by me yet), it implements services similar to zope catalogs and enables universal management of references (such as the Archetypes reference engine but in a more sustainable way). It is based on RDFLib, similarly to ROPE.

I feel enthusiastic about this product since it sounds to me like a good future-proof solution for the management of metadata, references and structured data within content management systems and portals. Plus Zemantic would sit well in my vision of Plone as a semantic aggregator.