Ever been editing a picture, and figured, "I got this neat tag X, but what else should be tagged with it?" Well you got it. highlight your tag, click on related tag, and it will fill in the related tag section with things other people have tagged with it, BAM! Tasty snazzy tag soup.
But wait, theirs more? Anything you've already tagged? Highlighted, making it much easier to find new tags to tag with.