Digg or Slashdot-Style Comment Ratings

Hey, I’m looking to have it to where people can give comments ratings. The setup would be much similar to Digg’s thumbs up/down, with the comment having an overall score depending on how many + / -‘s they get.

The idea is that the blog will be posted for people to critique their writing, but only registered people may comment (anyone can give a comment rating). If the comment gives some helpful criticism, people would give it +’s, obviously. If it’s not, of if it’s trolling, it gets downmodded.

There’s got to be a plugin that I’m missing to have that set up. Or maybe someone’s got a tutorial to do it in AJAX or something. Anyone?