CommentMilk will parse the URL's in comments, and also manages contests through comments.
CommentMilk is a plugin to handle feed processing in comments, as well as integrating a contest system.
When a reader submits a comment and enters a URL in their comment, CommentMilk will check to see if a WordPress feed exists. If a one does, CommentMilk grabs the latest post and will add it to the end of the users comment. What makes CommentMilk unique is the ability to give your readers the options of typing in a keyword to replace the latest post link.
CommentMilk can also help run contests. You can add "campaigns" that will add extra fields that you specify to the comment form. You specify the amount of points the reader should receive for filling out for filling out these fields, and the points are generated when the reader submits the form.