Test which version of a block on your site is most effective. For example, you could test different versions of a graphic that asks visitors to make a donation. Or, maybe you have two or more versions of a paragraph that you want to use to convince people to buy your product. Which one is most persuasive? Rather than rely on only your intuition, test your hypothesis with the Experimenter add-on. The add-on will automatically present a random block from a group of blocks that you choose. It will collect analytics and help you see which block is most effective for generating conversions.
A/B testing and Multivariate testing is used by many successful sites from Google to Obama's campaign donation page.
For a more detailed explanation of A/B testing, check out this Smashing Magazine article: http://www.smashingmagazine.com/2010/06/24/the-ultimate-guide-to-a-b-testing/