See RDF schemas for details of the schemes. This page lists applications only.
Metadata in PDF files
PDF files use the Dublin Core RDF scheme to include metadata.
Metadata in Images
Images (JPEG, TIFF, PNG and PSD) can include metadata in RDF. This standard is called XMP, which is a wrapper around and rdf:RDF statement.
I've seen frequent usage of at least the following namespaces:
xmlns:Iptc4xmpCore='http://iptc.org/std/Iptc4xmpCore/1.0/xmlns/' xmlns:pdf='http://ns.adobe.com/pdf/1.3/'></rdf:Description> xmlns:photoshop='http://ns.adobe.com/photoshop/1.0/' xmlns:tiff='http://ns.adobe.com/tiff/1.0/' xmlns:xap='http://ns.adobe.com/xap/1.0/' xmlns:xapRights='http://ns.adobe.com/xap/1.0/rights/' xmlns:mediapro='http://ns.iview-multimedia.com/mediapro/1.0/' xmlns:dc='http://purl.org/dc/elements/1.1/'
Beside XML, images typically also contain metadata in EXIF format. EXIF is not RDF-based (not even XML based). Roughly, EXIF contains automated data which is recorded by the camera, such as the date and exposure time. XMP on the other hand, contains higher level data, such as author and licence information.
A good starting point is http://www.adobe.com/products/xmp/standards.html, which has a great list of standardization bodies (including pointers to PDF, which also uses RDF for metadata). Great quote: One enabling technology is the Resource Description Framework (RDF) that utilizes Adobe XMP.. Ehh, guys: isn't it the other way around?!
Metadata for webpage
See the Creative Commons RDF schema. Licence information contains information about the restrictions on the redistribution of information on a website.
See the ICRA RDF schema. Content rating includes information about the applicability of a website for an audience. For examples, it allows a website author to say the contents contains foul languages, so that webbrowser can block the contents to be viewed by small children.
While P3P by default is specified in a regular XML file (using the p3p namespace), it can also be expressed in RDF. I'm not sure about the usage, but I have the impression it's not widely used.