Like digg, this Greasemonkey script allows you to rate comments.
As of now, the userbase is enough for ratings to have some meaning. However, the more people that use the script, the better & more useful the ratings, and thus the script, become.
Step 1: Install Greasemonkey
Firefox: Install this extension and restart Firefox.
Safari: Follow steps 1-4 of this tutorial.
Epiphany: Install epiphany-extensions using your package manager or via the link. Enable Greasemonkey under Tools > Extensions.
If you have instructions of another browser, let me know!