This might help @andygongea mashable.com/2015/01/30/ab-testing-tools Had this on my bookmarks!

Not necessarily A/B testing but I've used TryMyUI in the past. You give them instructions on what to do / test and questions for them to answer and real users go to your site, carry out your instructions and report back with a video recording. It's worked well for me.

As for A/B testing a webpage, I think you might need to roll your own script and present different versions of a page at different intervals to different users. I'm not sure you could use a third party service to present different versions of your site to users and still have it be functional. You'll need to collect metrics yourself on this, obviously.

A/B testing services for emails definitely exists - mailchimp, kissmetrics, campaignmonitor are a few examples of services that do it.