I’ve recently stumbled upon Responsivator, a nifty tool created by John Polacek that lets you "view & share how sites look at different screen sizes" through iframes, and I’ve been using it to test my web + interface design projects since.
Basically it saves you the hassle from having to manually check what your responsive site design looks like and how users would interact with your site on multiple common screen resolution and devices (iPhone, iPad, some android phones and tablets and the desktop / laptop). What’s really cool is that you can even hit the customize button to manually input in a screen size in order to test on the odd resolutions (eg. 240x320 HTC Tattoo).
So if you don’t need to test for touch-based functionality, Responsivator will save you a lot of time and money when you’re conducting tests! Alternatively, you could also use the view random button on Responsivator to find inspirations on responsive web design. However, nothing beats actually testing your site on a device.
Note - some viewport meta tags like width=device-width and initial-scale=0.8 are not recognized, so you’ll have to manually check on your target device.