We (Harry's) have released a tool called Kontrast: https://github.com/harrystech/kontrast, http://engineering.harrys.com/2014/09/24/kontrast.html

It's written in Ruby but is not really dependent on Rails or anything to run. We use it to compare a list of URLs between the version we are currently testing in CircleCI and our production site. Any differences are highlighted in a nice gallery and we get a message in our Hipchat room.