Short introduction to the semantic web by Manu Sporny from digitalbazaar.com.
RDF is a method for expressing semantic information. RDF/XML is probably the most common syntax for RDF.
RDFa is a set of extensions to XHTML that allows RDF to be encoded within an XHTML page. The wiki on this site provides links to several examples of RDF in use.
Google indexes the public Web for XHTML Friends Network (XFN), Friend of a Friend (FOAF).
Google tool for creating canonical URLs from the various forms you may enter for your account on a social networking site. Has a link to a demo site. (Note: sgn:// is Social Graph Node)
A microformat specification. The Getting Started link shows you how to add XFN to a web page.
Operator is an extension for Firefox that adds the ability to interact with semantic data on web pages, including microformats, RDFa and eRDF.