I created a bookmarklet which can resize your browser window to any of the common resolutions. It currently has all the common desktop, Android (both tablet and phone) and iOS screen sizes.
To use, drag the following link to your bookmark bar:
Now open any website and click the bookmark. A dialog will open showing you a list of screen sizes for you to click on. For the mobile screen sizes (currently Android & iOS), you can pick from Portrait or Landscape mode.
For interested developers: it's hosted on Github