This plugin is no longer maintaned.
This grabs the feed from your commenter's website and puts a little bit at the end of their comment that says "Recently (date) someone wrote 'blog post' at 'website'. Check it out!". As always, there's a handful of settings you get to tweak to make it work for how you want it to work, and it'll work just fine if you leave them at their default values. Please don't comment here as no one monitors this space. In the forums or on my blog works much better :)