The DiscussionPlugin adds discussion forums to Trac. An arbitrary number
of forums can be added, organised to forum groups. Users can create
topics in forums and reply to them which together creates discussion
threads. Threaded or flat view to topics and its replies is supported.
Each forum has a list of moderators who can delete topics, replies,
etc.  Appending new forums and topics either as new replies shows up in
timeline. Searching capability in topics and replies is supported too.

WWW: http://trac-hacks.org/wiki/DiscussionPlugin