Follow post plugins for WordPress

I’m running a Q&A site with wordpress. I am wondering if there is any plugins that add a “follow” button at the end of a post like Quora such that users can follow the post or follow the topic.

Thank you.

Example page with follow button:
http://www.quora.com/Is-Quora-planning-an-API

This plugin “allows commenters to sign up for e-mail notifications of subsequent replies.”

Subscribe To Comments Reloaded