In case you didn't know, you can do easily A/B testing to check what is the best design or message to use for your campaigns.

if you have several popup windows with WPSubscribers they will randomly be distributed, you just need to look at the analytics to see which ones is the best.

As it is a topic far behind the only technical tools, I will compile a few others tools and suggestion to make better A/B testing and explain why it is important.

One of the recent ressources I discovered
http://whichtestwon.com/

you could play and test yourself about big companies A/B testing and understand about how it is important.

we are using PrettyLinks pro to load balance some target links too.

if you know nice tools, let us know.