GRDDL is a technique for using XML/XHTML dialects such as microformats as CustomRdfDialects. If you give the address of a document in one of these custom dialects, the script linked from this page should get the RDF data out of it and display it (in N3/turtle syntax).
There are GRDDL Implementations in C, python, XSLT, and Java. The tabulator is implemented in JavaScript; it supports GRDDL by calling out to an online service. Native GRDDL support is on the todo list. g1.js is some work in that direction. See dev notes for details.