It is true. The only people resizing their browser windows are the web designers.

Me for instance: I do not proudly possess an iPad or any other tablet. I do not have an iPhone, nor a BlackBerry or any other Internet connected small device. (I do own an ancient Nokia though, from Before Time Was Time!)
So I have to resize my browser windows to test all media query break points. What else?

You could use this testpage with some pre-defined iframes: Matt Kersley’s tool. Whereupon @lensco tweaked that page (Simple responsive design test page) and, at last, points out this Responsive Design Test Bookmarklet. All very convenient for a quick glance.

There are several online emulators as well, like iPad Peek.

And ultimately Opera offers a smooth and stylish desktop Mobile Emulator. As they say ‘Do your mobile development straight from your desktop, and pair it with Opera Dragonfly for advanced debugging’.
The emulator comes with a launcher where infinite instances can be defined.

For this I added a new category: Handy!

Ever so smooth and stylish: that’s Opera for you. I always considered the Opera browser ‘beautiful but superfluous’, but they are some really busy and inventive group of people. Dev.Opera is a great place to sniff around.