CSS responsive design testing made easy. This tool is a mini in browser testing lab for simulating device resolution. It lets you see the popular resolutions all at once in both landscape and portrait mode. Provides resolution testing for smart phones, tablets (nook, ipad), laptops, etc. This tool also supports URLs at localhost, since the code is all client side. Enter your URL and click the Load button to try it out!
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Trouble Shooting:

If nothing comes up, check the JavaScript console for this error: "Refused to display document because display forbidden by X-Frame-Options."

That means the website you are targeting blocks being loaded in an iFrame. If this is your site, and you want to turn that off, change the http header "X-Frame-Options:" to have a value of something other than SAMEORIGIN. See this stackoverflow question for more details: http://stackoverflow.com/questions/6666423/overcoming-display-forbidden-by-x-frame-options

Responsive Design Test Bookmarklet: http://www.benjaminkeen.com/misc/bricss/ (this one is nice to have in the favorites bar and works great for me in Chrome)