We've written a few posts on our blog about our A/B testing framework.
Our event tracking system: http://www.thumbtack.com/engineering/building-our-own-tracki...
ABBA, our tool for analyzing the results: http://www.thumbtack.com/engineering/gambling-with-the-devil...
A script for using ABBA with Mixpanel: http://www.thumbtack.com/engineering/ab-testing-with-mixpane...