Group items matching

in title, tags, annotations or url

XFML lets you exchange hierarchical faceted metadata. It also lets you indicate topics in different published XFML documents are equal, thus allowing you to reuse indexing efforts. XFML borrows many ideas from Topicmaps, a format you should check out if you like the ideas behind XFML but are frustrated with its limitations (see http://topicmaps.org).