http://www.esearchclick.com
Jeremy Sussman a Google Employee says:
Figuring out the relative weights of the different factors in the rankings is one of the most difficult parts of our algorithm.
I can assure you that the last thing we want is for the business who hires the best SEO to win a better slot. But, the SEOs are, unfortunately, pretty good at what they do, and so sometimes they out smart us (hopefully, only temporarily).