GD Star Rating(...) is a fairly popular WordPress plugin for rating and voting on posts and pages.
Bobbing Wide have installed it on a number of sites
www.rowlandscastlewebdesign.com
We had a couple of problems with the initial installation (I’m surprised that it works on [wpmu]) and some unwanted placement of the stars (easily fixed with a [ clear] shortcode)
… but there’s a lot of functionality, so we’re not going to give up quite yet, even though the plugin’s documentation is rather old.