Easycommenting is a product that enables commenting on any content item. Rather dull, but the technology used to create it is very interesting. The skeleton is generated with ArchGenXML, but the content is zope 3 aware and can be adapted to use by any other object.

My main interest is to regard it as a documentation, as its implementation is clean and covers several areas of functionality that I'm very interested these days (z3 content in plone, adaptors, views, etc).