SIOC and Firefox: detect and identify
It is a browser extension (for Firefox only by now) which inspects web pages for links to Semantic Web metadata and informs the user about their presence by showing an icon in browser’s status bar.
Originally developed to detect presence of SIOC metadata it has been extended to cover other types of data. Currently it supports SIOC, FOAF and DOAP metadata.
The current version of Semantic Radar also pings the Semantic Web Ping Service when metadata is detected. This allows a community based discovery of the semantic web data and a notification of a central ping service.
UPDATE: Fred also has a post on Semantic Radar