How to display 'new' by unread forum posts (like drupal.org)?
You can do this by a custom module. 

Create a new table in database with 3 column [ nid | uid | viewed_time ] representing node view history. 

And override hook_node_view(). The logic could be something like:

if (user is authenticated) {

  if (the node type is forum &amp;&amp; nid with current uid is not in a row of view history table) {

    append 'new!' to node title

  } else {

    write [nid , uid, current time] to the view history table

  }

}