Phantom Responsive.

phantom-responsive.js

var async = require('async');
var sizes = [
    [320, 480],
    [600, 1024],
    [1024, 768],
    [1280, 800],
    [1440, 900]
];
var url = 'http://one-pager.dev/nl';

function capture(sizes, callback) {
  var page = require('webpage').create();
  page.viewportSize = {
    width: sizes[0],
    height: sizes[1]
  };
  page.zoomFactor = 1;
  page.open(url, function (status) {
    var filename = sizes[0] + 'x' + sizes[1] + '.png';
    page.render('./screenshots/' + filename);
    console.log('I\'ve created /screenshots/' + sizes[0] + 'x' + sizes[1] + '.png');
    page.close();
      callback.apply();
    });
}

console.log('Hi, welcome to the responsive webpage tester.');
console.log('Let me create some screenshots for you!');

async.eachSeries(sizes, capture, function (e) {
    if (e) console.log(e);
    console.log('Done creating the screenshots!');
    console.log('Have a nice day!');
    phantom.exit();
});

Needs phantomjs and async.