UPDATE 1: We've switched over to CrossBrowserTesting in which to bring you totally free access forever for testing your Pens.
UPDATE 2: We no long partner directly with either of these companies. They are both perfectly good apps for testing your work in different browsers, but there is no one-click solution or free testing.
BrowserStack is an excellent tool for cross browser testing. You can open any URL in a huge array of different platforms and devices and browser versions. And not screenshots either, live testing with developer tools. And it happens right in your browser!
You need an account to use it, but the time savings is absolutely worth it. It's getting impractical to own the zillion devices you need to test on and maintain the zillion VM's in order to run simulators and old browsers and OS's and all that.
We now offer a one-click "Open in BrowserStack" button on our Full Page views: