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.

Stefano Lissa

Comment Notifier lets user to subscribe the comment thread of each single post of your blog. It improves traffic and loyalty.
Notification emails are fully customizable (both plain text or html) with a number of “tags” to insert author name, post link and title, receiver name and so on.
The “subscribe me” checkbox in the comment form area can be added automatically or manually.
Read more on <a href="http://www.satollo.net/plugins/comment-notifier" rel="nofollow ugc">Comment Notifier</a> offical page.
<h4>Other plugins</h4>
<ul>
<li><a href="https://www.satollo.net/plugins/hyper-cache" rel="nofollow ugc">Hyper Cache</a></li>
<li><a href="https://www.thenewsletterplugin.com" rel="nofollow ugc">Newsletter</a></li>
<li><a href="https://www.satollo.net/plugins/header-footer" rel="nofollow ugc">Header and Footer</a></li>
<li><a href="https://www.satollo.net/plugins/thumbnails" rel="nofollow ugc">Thumbnails</a></li>
<li><a href="https://www.satollo.net/plugins/include-me" rel="nofollow ugc">Include Me</a></li>
</ul>

Comment Notifier

Add comment subscriptions to the blog comment form.

Override Comment Deadline vs Comment Notifier