Originally known as XDB, this C++ library (both URLs refer to the same project) is for manipulating DBaseIII (.DBF) files.
It has been renamed to Xbase -- possibly to avoid confusion with another XDB. There appear to have been a number of efforts under the XDB name, mostly involving XML or object databases. There may have been a distributed database effort known as XDB (see discussion on SemanticWebIsStupid), but I can't find any reference to it, unless Oracle's XML DB (http://www.oracle.com/technology/tech/xml/xmldb/index.html) -- known as XDB at one point, I think -- is what it has evolved into.
Related: