Dublin Core

A "standard" set of Meta Data tags for HTML documents. No RDF, XML, or other fancy stuff required.

Used surprisingly little outside academic environments.

http://dublincore.org/ The Dublin Core Metadata Initiative is an open forum engaged in the development of interoperable online Meta Data standards that support a broad range of purposes and business models. DCMI's activities include consensus-driven working groups, global workshops, conferences, standards liaison, and educational efforts to promote widespread acceptance of metadata standards and practices.

http://dublincore.org/documents/2001/04/12/usageguide/

There is an online form/service to generate Dublin Core tags for you to copy/paste into a given document.