Simantics is one of the most sophisticated open source project-based tools I've seen. It reminds me of tools developed by Dassault Systemes.
Kune is based on Apache Wave (formerly Google Wave) and is similar to Rizzoma. This relation and further relations may be found on Wikipedia under Apache Wave.
Cytoscape is a wonderful visualization tool from the Bioinformatics community. It allows for RDF visualization and SPARQL Queries.