ok, i agree this is not exactly what it does but this plugin enables review features to your comments that allow rating of posts based on various categories. its an extension to dan grossman's wp-dg-review-site and adds category based rating categories and templates as well as simple star rating visualization and fraud protection. it also stores the overall score of a post in a custom field which allows custom queries to sort by rating or create top lists.
download and infopage for this plugin can be found here
you can see the script in action here