This app couldn't be simpler to use, just enter in the viewport widths you'd like to view and that's it. Multiple views so you can make sure your responsive website is working properly. I wished I knew about this sooner.

excelent for view responsive layouts, I use for bootstrap with this breakpoints: 320, 575, 576, 767, 768, 991, 992, 1199, 1200, 1399, 1400