Are you looking for something unmoderated? If you have clearly defined goals that you want to subject to different design options, Visual Website Optimizer facilitates “in the wild” experiments quite nicely. Of course, this assumes you have enough traffic to make it significant.
Google Analytics is also an option but it’s got more setup overhead for you define the steps in your goals. And it won’t capture time to complete a task very well, out of the box. But it’s free and can do many useful things. Again, this assumes you have enough traffic.
For moderated experiments, your options are almost infinite.