Very similar to metadata features in ordinary HTML:
<meta name="dc:creator" content="Mark Birbeck" />
<link rel="foaf:depiction" href="http://www.flickr.com/my-picture.png" />
<a rel="foaf:knows" href="http://www.spy.co.uk/nico.html">
Nico's home page
</a>
<span class="bib:book">
Canteen Cuisine
</span>
Adds @about so we can talk about other things:
<a rel="foaf:knows" href="http://www.spy.co.uk/nico.html">
Nico's home page
</a>
<div about="http://www.spy.co.uk/nico.html">
...
</div>
Adds @property, like @rel but for text:
<a rel="foaf:knows" href="http://www.spy.co.uk/nico.html">
Nico's home page
</a>
<div about="http://www.spy.co.uk/nico.html">
<span property="foaf:name">Nico Macdonald</span>
</div>
Adds @content, to clarify text:
<div>
We are meeting
<span content="2007-06-27">today</span>
at Frontline.
</div>
Adds @datatype, to further clarify text:
<div>
We are meeting
<span content="2007-06-27" datatype="xs:date">today</span>
at Frontline.
</div>
Adds @role, to explain purpose of mark-up:
<div role="footer">
Copyright (c) 2007
</div>
<div role="wai:toolbar">
...
</div>
Important for accessibility.
Put metadata into any HTML/XHTML document, even a blog entry:
<span about="urn:ISBN:0091808189" class="bib:book">
Canteen Cuisine
</span>
Joost using this technique for videos, and adding country, language, rating, etc.
Ruby on Rails extension to make it easy to publish RDFa.
IBM has a number of things that bring in the metadata from documents and uses them in other processes.
Operator Firefox extension, to rip vCards and event details from documents. (Supports both microformats and RDFa.)