Coronoro

Simple Plugin which shows whether a registered user has read a Post or not. 
You can also choose to show the number of unread Posts at Forum level. 
This Plugin is an update to the older <a href="https://wordpress.org/plugins/bbpress-unread-posts/" rel="ugc">bbpress-unread-posts</a> Plugin.

bbPress Unread Posts v2

Simple Plugin which shows whether a registered user has read a Post or not.

destroflyer

Shows a read/unread icon next to threads (Marks topic as read when the user visits it). 
A “Mark all topic as read” button on each page allows to set the topics on that page as read. 
Developed for: http://jmonkeyengine.org

bbPress Unread Posts

Displays a read/unread icon next to threads and offers a "Mark all topics as read" button for each forum page.

bbPress Unread Posts v2 vs bbPress Unread Posts