Google Website Optimizer is kaput, replaced with Google Experiments. With Content Experiments, you can compare how different web pages perform using a random sample of your visitors, define what percentage of your visitors are included in the experiment, choose which objective you'd like to test and, eventually, you'll also get updates by email about how your experiment is doing.
Here's a video overview of Google Experiments:
As sophisticated and expansive as Google Analytics has become, I'm actually having a tough time with Google Experiments. It's just my opinion, but I really don't understand why Google went at this from a page URL level. Most companies utilize content management systems nowadays and it's a pain to set up multiple pages, block each from search engines, and develop a means of driving traffic randomly between them.
Most web pages are developed, not as an entire unit, but as a page with different images, content, sections, buttons, call to actions, etc. Google discusses content optimization on the Google Experiments page, but while you can iterate through changing some elements on the published pages, it would be so much nicer if I could just test elements rather than the entire page itself.
Have you put Google Experiments to work? Am I missing something?