Easiest acceptance testing framework I've used is Splinter. It's an abstraction layer over Selenium that practically makes writing tests similar to plain English: http://splinter.cobrateam.info/
Still, it is super flexible, and I enjoyed using is a lot. Nice feature is having executable requirements if BDD is used. I.e. you can implement keywords to execute given-when-then req's and they became your test.
[1](http://robotframework.org/) [2](http://code.google.com/p/robotframework/)
For .NET users, FluentAutomation (http://fluent.stirno.com/) is a good option. Works in regular tests or with scriptcs .csx files. Can automate any of the Se supported browsers, including PhantomJS very easily with short, readable commands.