As far as I know, it is not possible to know whether a site provides an RDF feed unless it is either driven by Slashcode (where the feed is provided at http://www.website.org/website.rdf as in the case of Slashdot) or Manila (where the RDF-like RSS feed is available from http://website.com/xml/rss.xml).
xmltree.com is a good directory of various news feed formats, and there is an excellent weblog which is a Manila site, and so has an RSS feed here.
While I'm sure some other content management systems provide feeds, I'm not aware of the default addresses for them.
Slashdot Mirror
As far as I know, it is not possible to know whether a site provides an RDF feed unless it is either driven by Slashcode (where the feed is provided at http://www.website.org/website.rdf as in the case of Slashdot) or Manila (where the RDF-like RSS feed is available from http://website.com/xml/rss.xml). xmltree.com is a good directory of various news feed formats, and there is an excellent weblog which is a Manila site, and so has an RSS feed here. While I'm sure some other content management systems provide feeds, I'm not aware of the default addresses for them.