This is a must-have bookmark for everyone in interactive marketing:
"MarketingExperiments.com is a division of Digital Trust. It's an online laboratory with a simple (but not easy) five-word mission statement: To discover what really works. The Lab tests every conceivable marketing method on the Internet."
Some of their recent reports are on click fraud, email capture, AdWords, and free trial offers. Much of the Lab's research is proprietary, but they do release a fair portion of it on their website for free.