This was a great web tool that allowed you to see only the negatives and neutral feedbacks left for an eBay member. All you had to do was enter the username, and it would return negative and neutral feedbacks only. I always look for patterns in feedback. If the comments are constantly about poor grading, poor packing, etc, I suspect there is a problem with the seller and I avoid. It's usually fairly easy to detect a "revenge" feedback and I typically ignore them.
The feedback lookup tool was especially helpful with sellers that have a lot of negatives buried under a mountain of positive feedback. Bear in mind that this is a common tactic used by sleazy sellers -- they can shill their own auctions and leave positive feedback to quickly move a negative comment off the feedback front page because they know many eBay users won't bother to go back more than a page or two.
I will keep an eye on this site... hopefully they will be back up soon.