Google Website Optimizer

A/B testing comes to the masses with WordPress plugin

Multivariate testing is one of the most powerful tools available to
online publishers. But many of them don’t use it for various reasons,
from lack of knowledge about multivariate testing to lack of simple
testing solutions.

At a weekend hackathon event, a couple of developers decided to change
that by building a Headline Split Tester WordPress plugin that gives
WordPress publishers the ability to set up A/B testing of their post