A/B testing is a really interesting activity - trying out two different designs simultaneously, checking success rates and going with the best one.
Google have an great free tool - Website Optimizer - and it's just a shame that we use a content management system that stops me from using it. I have experimented with it when using HTML prototypes, but it's not the same. You really need a decent number of visits to each version, and real people using a real site is where the real benefit comes in. Prototypes don't really cut it in this case.
Google's Website Optimizer - A free A/B split & multivariable testing tool to identify your most effective site elements
A/B testing examples
Anne Holland's Which Test Won is a great site if you can't try out A/B testing yourself. It's also good fun. Take a look at two versions of the same page and try to guess which one came out on top when tested.
Anne Holland's Which Test Won website