Thanks to Tim Wilson for pointing this out to me in a comment on a previous post.

For those who lack access to a statistical software solution that will also interpret the results, A/B testing is challenging at best. Testing for statistical significance is important, but just as critical is an understanding of your tests' validity and how to use the results to improve future campaigns. To answer these needs, Bulldog Solutions offers two Excel-based A/B calculators. You can view a demo or download these calculators here.