This is a great little tool. It allows visitors to choose whether or not a commenter helped them. A vote of yes will raise that commenter’s rating. A popup shows the results of the commenter’s past comments. You can see how it works here.
Go to one of my comments. If you hold your mouse over the "Profile" link after my name, you can see where I commented in the past. The yse/no buttons at the end of the comment let you, the reader, choose which comments helped you most.
It is a great concept which I hope takes off. Not tool many blogs are using it. I ran into it over at Read/Write/Web. They have plugins available for WordPress and TypePad. You have to sign up first and get an access key. They have a great tutorial over at SezWho with screenshots and if you run into issues, the forum support is fast enough. I had a minor issue and it was fixed quickly.