A very, very, interesting new product from Google just launched: Google Website Optimizer

They boast that it can perform multivariate tests of landing page content in order to increase website conversions. In non-marketer speak, that means that you can send people to different versions of your website’s landing page, each with a different mix of text and graphics. You can then track whether or not you got a purchase/registration/subscription from each person who landed, and thus quantify the added benefit of each different combination of web page pieces.