Great examples of experiments are available at Marketing Experiments journal (9),
Design Choices Can Cripple a Website (10),
Call to Action (11), and Which Sells Best (12).
Forrester’s Primer on A/B Testing (13) mentions a few good examples of positive ROI.
In shop.com’s The State of Retailing (14),
the authors wrote that in their survey of 137 US retailers "100% of the retailers that employed usability testing and A/B testing of offers and promotions rank these tactics as effective or very effective".