Optimizely┬« is a Digital Experience Optimization Platform that helps you customize user experience and perform feature and A/B testing on your website among other things.
Feature testing let’s you rollout a feature selectively and target audiences based on browser, location and several other factors, before you plan to go mainstream with your new feature.
A/B testing is testing out multiple versions of the same website which can be compared for performance and multivariate testing.
Optimizely┬« let’s you customize and rollout these changes through multiple ways, checkout this tutorial for web interface customization and checkout this tutorial for an SDK based customization of Oslo.
For more information on Optimizely┬« check out: https://www.optimizely.com