Google's purchase of web analytics firm Urchin 18 months ago is paying off for website operators today as Google unveiled (a beta version of) Google Website Optimizer at the eMetrics summit in Washington.

The free analytics product is designed to address the other side of Google's web advertising business. Google's made a huge amount of money selling advertising to drive users to advertisers' sites. Google's role had ended at the click-through. You pay for the clicks and once they click through, it's your job to conver 'em.

GWO helps website operators do the conversion by setting up several different variations of landing pages, having Google rotate the different combinations and generating reports on how the different combinations performed. A conversion, of course, doesn't have to be a sale. More likely, for a small e-commerce site, it's signing up for email or providing contact information.

For small business websites, this is invaluable information that there has really been no way to get at. They will gobble it up. I'm no web analytics expert but I suspect that most any business that is spending money on AdSense will need to be using GWO to constantly tweak their ad messages and landing page content.

But there is a catch. With GWO, Google actually serves as middleware. You put code on your pages that as far as I can understand alerts Google that it should intercept the serving of the page and offer up one of several variations instead. That means that not only do you know more about what works - Google does too. And for a huge number of sites.

But perhaps someone else can post about exactly why that's problematic.