Updated Oct 3 2008:

C++ engines are often neglected for web-based inferencing applications, but compilation is possible on non-shared hosting ...

Updated: Sept 24 2007

It is a browser plugin, which introduces browser dependencies, but the project is Open Source and seems to be active and well-supported.

From the site:

Mozilla RDF was originally used to support the Aurora/Sidebar user interface and SmartBrowsing metadata services. It's main use in Mozilla now is as a common data model and API for use in XUL-based applications.