Google's AdWords Campaign Experiments or ACE (a free tool for testing and measuring changes to keywords, bids, ad groups and placements) announced in early August 2010 is now rolling out globally.
What will be appealing to advertisers is that experiments can be set up to measure the impact of changes with just a small percentage of your traffic. ACE runs experiements side-by-side (split testing) with the existing campaign and provides indicators of where performance is significantly different.
The potential uses are pretty broad. For example, you could add new keywords and measure that tactic's affect on overall campaign clicks, costs and conversions. Or you could test increasing or decreasing bids on high value keywords, or test new landing page designs.
Keep in mind that ACE is designed as a split testing tool - e.g. testing two different landing pages for a keyword, ad ad, or ad ad group directly in Adwords. To test more than two (multivariate testing) landing page designs , to test a landing page with multiple traffic souces or test multiple elements on the landing pages, you should still use Google Website Optimizer or one of the many other testing solutions including Adobe Test & Target, Amadesa Customer Experience Suite, Autonomy Optimost, Maxymiser Content MVT, SiteSpect, Vertster Conversion Optimization Suite, or Webtrends Optimize.
To get started, sign into AdWords and access ACE on the Campaign tab under Settings. For those outside the U.S., this is a rolling launch so you should see it within a week.
As a side note, ACE is supported in the newest AdWords API version announced today.