What is your favorite tool for A/B testing and optimization?

A/B testing is all the rage now. There are lots of choices: Google Website Optimizer, Mixpanel, KISSmetrics, a number of ones that live inside your application, and more. What is your A/B testing suite of choice, and why?

Hey Mark! You're probably more versed in A/B testing than I am, but I find Google's tools to be sufficient - for a couple of reasons.

A) It's pretty powerful
B) If we spent all day analyzing things from 10 different angles, we'd never get anything done.

I haven't found a scenario where Google didn't do the job (though not always as elegantly as it could) and it's free.

The biggest problem with A/B testing is that it's very difficult to account for any other influences on the results. Most companies don't have the patience to change just one thing at a time and let the results pan out, so they end up with a bunch of inaccurate data.

I'm sure other will chime in with more detailed insight.