In the past if you wanted to use Google Website Optimizer you had to sign up for AdWords account. That is no longer the case.
It is now a standalone product. It is a free website-testing tool. You can gain access to Website Optimizer with any Google Account. You can test which designs, headlines, and graphics prduce the highest conversion rates.
You can learn a lot more about this with official information about Google Website Optimizer direct from Google
Official Google Website Optimizer Blog: Content Testing Goes Mainstream