Thread Description Language (TDL)
TDL is an RDF vocabulary for desribing threaded discussions, such as Usenet, weblogs, bulletin boards, and e-mail conversations.
So what could it be used for? One obvious thing would be to enable client software to access web based message archives without having to care about scraping the HTML to see if all the header information is there - just embed the RSS in a page,
Another would be as a uniform way of storing meta data about messages and their relationships, or exchanging that information with other applications.