This is a pretty cool investigative tool for open source collaboration. From the Poynter writeup:

A ProPublica reporting project published today turns primary source documents into a platform for crowdsourcing and reader collaboration.

The investigative reporting nonprofit built a tool called DocDiver, a plugin for DocumentCloud that creates an annotation layer on top of document pages. Readers can make notes on the document, and journalists and other readers can see those notes threaded in a sidebar.