In my previous company, I was evaluating a bunch of open source and commercial tools and we zeroed on "cross browser testing" online tool. The beauty is it supports 100+ browsers including desktop and mobile and it has interactive and side-by side. But its not free. We chose browsers and paid accordingly. It also points out the UI/functionality inconsistencies between browsers, which is awesome!
http://crossbrowsertesting.com/
I've never used "cloud testing" service (not free) but heard its good too. http://www.cloudtesting.com/
If you want a free tool you could use "browsershots" which supports 65+ browsers, but its does have interactive testing. Sauce labs is also good in that it allows you to record scripts in Selenium and lets you run the tests on various browsers.
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