P2 Resolved Posts is a WordPress plugin by danielbachhuber and automattic.
Mark a thread as “unresolved” when there’s something needing to be finished, and mark it as “resolved” when it’s been finished. Sidebar widgets allow you to see all unresolved posts, optionally filtered to a specific tag.
P2 is a great tool for communication and collaboration. P2 Resolved Posts makes it easier to better understand when a discussion has reached completion.
Download: http://wordpress.org/extend/plugins/p2-resolved-posts/