Document Cloud is a platform that is primarily meant to be used by journalists who are reporting on (or publishing) primary source documents. They will be able to have their documents run through OpenCalais, and get comprehensive information on all the places, people and organizations that are mentioned in the document. Plus, Document Cloud can look at all the dates that are mentioned on the original text, have them plotted on a timeline, and let them know about documents that are related to what they have written.
Additionally, reporters will be able to annotate their documents with both public and private notes. That will make it very easy for them to organize their thoughts, and also to point readers to specific portions of their texts.
And those who decide to make their documents public through the Document Cloud website will be enabling people from all over the world to use and quote them on their own sites (and blogs). This public catalog already has thousands of documents that can be freely accessed.
DocumentCloud.org In Their Own Words
Analyze, annotate, publish. Turn documents into data.
Why DocumentCloud.org It Might Be A Killer
It gives journalists an excellent framework for doing research and sharing their finds with just everybody.
Some Questions About DocumentCloud.org
Which other uses can this potentially have? 
