Soprano (formerly known as QRDF) is a library that provides a highly usable object-oriented C++/Qt4 framework for RDF data. It uses different RDF storage solutions as backends through a simple plugin system. Soprano is targetted at desktop applications that need a RDF data storage solution. It has been optimized for easy usage and simplicity.
Nx2 is a somewhat complex Web application that acts as a frontend to music databases. Based on XUL, it runs in Gecko based browsers such as Firefox. Built on this idea is a client/server model where several Nx2 frontends can connect to each other, creating a way to link together databases on different networks.
4store is an efficient, scalable, and stable RDF database. It was developed to underpin Semantic Web applications. It has been providing a base platform for these applications for around 3 years. It has successfully held and run queries over databases of 15GT, supporting a Web application used by hundreds of thousands of people.