Julien Sobrier of Zscaler delivers a fascinating video presentation to Google as part of their "Tech Talks" series on Black Hat Spam SEO, which is the process of using Search Engine Optimization (SEO) techniques to get links to malware sites to rank highly on Google search results pages:

One statistic that really struck me from the presentation was the fact that more than 50% of popular search result pages on Google had at least one SEO link in the first ten results.

Julien writes for Zscaler on their cloud security blog here, which is well worth adding to your feed reader:

https://www.zscaler.com/blogs/security-research

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Updated 2021 : note that the above post was originally published in 2010, but is left here for archival purposes. I have fixed the external link, and embedded the video directly from YouTube.