Yet another interesting take on XML metadata representations: FacetMaps. A facet map (as I understand it) is a way of combining facets with hierarchies, best explained by the excellent interactive three minute concept intro on the site. One of the main contrasts to XFML is that in a Facet Map Facets, rather than Topics, are the principle categorisation element. A resource in a Facet Map is linked directly to one or more facets, rather than going through a topic. The XML format is pretty simple (a lot simpler than XTM and XFML) so I might have a go at a PHP implementation at some point.