2012
The Open Graph protocol
by jdrsantos & 1 other2011
RDFa API
by Xavier Lacot2009
Bill de hÓra: Snowflake APIs
by greut (via)RDF is worth learning for a different reason — the profound enlightenment experience you will have when you finally get it. That experience will make you a better format and data API designer for the rest of your days, even if you never actually use RDF itself a lot. (You can get some beginning experience with RDF fairly easily by writing and modifying simple files like FOAF and DOAP for social networks and software projects, or RDFa extensions for XHTML.)