Without diving into the arcane specifics, I can safely say that I wouldn't have my sanity today without Optimizely.
It's mind-numbingly simple to use, but also hugely powerful, and intelligently implemented. Many people seem to assume the power/appeal is in the visual editor, but they've advanced so far ahead of just being "visually created experiments", in both experiment design, targetting, implementation, and customization; they've entirely eliminated the headaches and fudging that usually comes with online testing tools. I'm as happy of a customer as they come.
It's mind-numbingly simple to use, but also hugely powerful, and intelligently implemented. Many people seem to assume the power/appeal is in the visual editor, but they've advanced so far ahead of just being "visually created experiments", in both experiment design, targetting, implementation, and customization; they've entirely eliminated the headaches and fudging that usually comes with online testing tools. I'm as happy of a customer as they come.