OpenLink Virtuoso Universal Server is an open source, web-based and command-line software project that provides a virtual database product, which offers transparent, real-time access to disparate data sources, such as ODBC, XML, Web Services and JDBC, as well as transparent integration of disparate application logic like J2EE integration and Mono ECMA-CLI.

OpenLink Virtuoso Universal Server's broad protocol support enables it to offer web, file, and SQL database server functionality alongside native XML storage, and Web services platform functionality as part of a cohesive single server solution. It supports SPARQL embedded into SQL, which can be used for querying RDF data that is stored in Virtuoso's database.

It features a unique hybrid server architecture that comprises of relational data management, XML data management, RDF data management, linked data server, document web server, text content management, web application server, full-text Indexing, as well as web services deployment.

OpenLink Virtuoso Universal Server is a cross-platform software that can be used on all mainstream operating systems, including GNU/Linux, Microsoft Windows and Mac OS X. It is written in the C and PL/SQL (Procedural Language/Structured Query Language) programming languages and designed to run on a console/terminal environment.

The software is available for download as a universal sources tarball that can be installed on virtually any Linux kernel-based operating system, as well as on Mac OS X, BSD, Solaris and Microsoft Windows OSes. It can be easily administer using its powerful and user-friendly web-based interface. Both 32-bit and 64-bit instruction set architectures are supported at this time.

This is the open source edition of OpenLink Virtuoso, but the project is also distributed as a commercial edition that includes even more features and paid support for small and medium-sized business. Prices start from 100 bucks for the latest version of OpenLink Virtuoso for Linux, Macintosh or Windows platforms. See the project’s homepage (link below) for more details.