Taggregation application++. Shadows is yet another tagging application. Unlike, David Carpe, who pans the initial unveiling as agitprop, to me it looks like there's about 1/2 a new idea in there. Gathering all the information, (tags, comments, and ratings) about a particular URL into a Shadow page is a new twist on tagging apps, at least to me.
But Carpe's right, it's a pretty crowded dance floor and a I don't see a lot of reason users would pick Shadows to boogie with.