GD Star Rating is advanced and feature packed plugin for setting up rating system on your WordPress blog. Plugin allows you to set up different rating systems for posts, pages and comments with great degree of customization.
List of features is very, very long, and you can check it out on the page on the right, or better yet go to the official website. Plugin will still remain free and there is no intention for that to change. But, support terms are changed and here you can find more info on that.