The Schematron namespace URI references a RDDL directory related to the Schematron.
The XML Schema namespace URI references a RDDL directory related to the XML Schema.
The Examplotron namespace URI references a RDDL directory related to the Examplotron Schema specification.
The Hook namespace URI references a RDDL directory related to the Hook Schema specification.
The Connect 0.1 is a utility language and toolkit for wiring together programs that are made up of XSLT transformations. The use of a higher level language allows, in the simple implementation here, you to hide the invocation details of different implementations of XSLT.
The dictonaries in http://webns.net use RDDL as the XHTML format. Some of these are:
XSet an XML property set production language.
XMTP an RDF/XML mapping of MIME/RFC822.
XSLTunit XSLT unit testing.