Google Website Optimizer

http://blog.kissmetrics.com/guide-google-website-optimizer/
http://searchengineland.com/how-to-segment-google-website-optimizer-tests-33341
http://www.grokdotcom.com/2007/04/03/64-tips-for-getting-started-with-google-optimizer/
http://www.blastam.com/blog/index.php/2009/03/top-50-google-website-optimizer-tips-on-what-to-test/