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There is a similar tool built into Firefox (Tools -> Web Developer -> Responsive Design View)

Didn't know that. That's pretty cool. However, its only for Firefox 15 which isn't available for mac yet..

It is available for Mac! The latest available version is Firefox 21.0 (stable channel).

What do you know. I updated Firefox from Firefox itself and it only updated me to 12.

Google Chrome has it built into the inspector as well, however it is not nearly as fancy as the one found in Firefox.