Recently, I was chatting to a client about responsive design and testing. I mentioned something I'd been using locally for a while and figured they'd find it useful. So here's a little something I use when I'm building websites and landing pages. I hope you find it useful.
You can see an online working version at http://www.responsivetester.net - you'll also find some more instructions there.
It's all MIT licensed, so feel free to use as you please. That said, if you find it useful, I'd appreciate a link and I'd love to see what you've made :-)