From Optimization at the Obama campaign: a/b testing:

Optimization was the name of the game for the Obama Digital team. We optimized just about everything from web pages to emails. Overall we executed about 500 a/b tests on our web pages in a 20 month period which increased donation conversions by 49% and sign up conversions by 161%. As you might imagine this yielded some fascinating findings on how user behavior is influenced by variables like design, copy, usability, imagery and page speed.