The Unanswered Discussions block is a quick and easy solution to finding and addressing the discussion forums in a course that you’ve not replied to. If you’re managing a course and want to be sure to reply to each discussion post or thread in a given course, this provides a shortcut in order to do so. The student created block is now available for Moodle 2.0 through 2.7 and is now maintained by Michael de Raadt (@salvetore).
Settings allow you to omit some forums and choose the number of each displayed type (random, oldest, user’s).
Read more about it http://docs.moodle.org/26/en/Unanswered_Discussions_block