Split Testing is a tool that allows you to calculate the statistical significance of an advertising split test. Just enter in the number of clicks and CTR you got on two different ads and Split Tester will tell you how likely it is that you have a statistically significant winner. This is not an affiliate link.

Free A/B Split Testing - Increase site performance | Joe Bennett, Chicago SEO

There is also a paid upgrade but here's what they say:

Definitely worth trying out.