Mangrove: An Evolutionary Approach to the Semantic Web "The Mangrove project seeks to create an environment in which users are motivated to create semantic content because of the existence of useful semantic services that exploit that content and because the process of generating such content is as close as possible to existing techniques for generating HTML documents. Our goal is to facilitate the simple annotation and subsequent extraction and querying of the enormous amount of information that already exists within the WWW's billions of HTML pages, rather than requiring the creation of new content from scratch. Our approach thus seeks to faciliate the gradual transformation of the current web into the semantic web."
Very similar to the ideas expressed recently in "interview followup". Semantic Tagger is an interesting example of this (the schema is an XML schema).