Responsivator – Findout “How Sites Look At Different Screen Sizes” Easily
Responsivator is a free to use web app (it is also open source) for quickly viewing websites with different screen sizes.
Once a URL is entered, it is loaded inside iframes with popular screen sizes (desktop, tablet, phone) in different orientations.
It is possible to customize the screen sizes and add/remove any new ones.
Also, each website tested gets its own URL to share it with others (an option exists for hiding the header of the app while sharing).
Responsivator can easily become a part of the "testing stack" of any designer/developer working on responsive layouts.