Scott Nelle

WordPress lets you set a number of days after which comments will be unavailable for individual posts. This plugin will allow you to keep commenting open on specific posts.
Once the plugin is enabled:
<ul>
<li>Set a comment deadline in Settings > Discussion using the “Automatically Close Comments…” option. Closing comments on old posts is a great way to avoid spam comments.</li>
<li>Edit an individual post on which you’d like to keep discussion open indefinitely and check the Override Comment Deadline box.</li>
</ul>

Override Comment Deadline

Allows you to enable comments on posts which are older than the comment deadline set in discussion settings.

Hugh Lashbrooke

This plugin is built for and requires <a href="https://github.com/Automattic/o2" rel="nofollow ugc">O2</a> to be installed.
This plugin allows you to mark a selected comment as the one that effectively sums up the conversation from the thread. This is great for disucssions that require a final decision to be made, support threads where one comment has the best solution, and a host of other uses.
Functionality includes:
<ul>
<li>Marking a chosen comment as the ‘top comment’</li>
<li>The top comment is displayed at the top of the comment list with a ‘view in context’ anchor link</li>
<li>The top comment is also highlighted in context in the thread</li>
<li>Only one comment can be selected as the top comment</li>
<li>The top comment flag can be removed</li>
<li>Only users who are able to edit the post can select a top comment</li>
<li>Includes basic styling for top comments</li>
<li>‘Top comment’ label can be translated and/or filtered</li>
</ul>
<a href="https://github.com/hlashbrooke/The-Final-Word" rel="nofollow ugc">Contribute on GitHub</a>.

The Final Word

Have the final word in a comment thread by marking a chosen comment as the 'top comment'.

Override Comment Deadline vs The Final Word