MyLongTail
Most Web tools today isolate you from the data you need to improve your website from a natural-search point-of-view. By utilizing a radically improved method for managing long lists on the Web, MyLongTail zeros right in on that data, showing you every search hit that has led to your site and then pulls out the keywords. MyLongTail then uses sophisticated algorithms to pull out underperforming terms as suggestions, which you may or may not choose to move into your to do list and act on. The more you take action by writing about these promising topics, the more your site snowballs.
(free while in beta)
UPDATE 7/3/2006
MyLongTail now is HitTail
☍ 03.06.2006