演示地址：http://www.jq22.com/jquery-info1799
jquery插件，使用验证过可用。
分析源代码后总结原理：设置background样式为fixed，判断浏览器滚动距离，当在视窗范围内时，调用$(window).scroll。并根据所设置的速度比例，进行同步滚动。
调用时，设置三个必要参数：data-speed，data-type，background样式为fixed
<div data-speed="4" data-type="background" style="background:url(...) 50% 0 no-repeat fixed">
js会根据data-speed来计算滚动速度，越小越快。
可以根据不同的data-type作不同处理，可自定义写代码，很方便。
background必须是fixed。
parallax.js源代码（需先引入jQuery）
$(document).ready(function(){// Cache the Window object$window = $(window);// Cache the Y offset and the speed of each sprite$('[data-type]').each(function() { $(this).data('offsetY', parseInt($(this).attr('data-offsetY')));$(this).data('Xposition', $(this).attr('data-Xposition'));$(this).data('speed', $(this).attr('data-speed'));});// For each element that has a data-type attribute$('section[data-type="background"]').each(function(){// Store some variables based on where we arevar $self = $(this),offsetCoords = $self.offset(),topOffset = offsetCoords.top;// When the window is scrolled...$(window).scroll(function() {// If this section is in viewif ( ($window.scrollTop() + $window.height()) > (topOffset) &&( (topOffset + $self.height()) > $window.scrollTop() ) ) {// Scroll the background at var speed// the yPos is a negative value because we're scrolling it UP! var yPos = -($window.scrollTop() / $self.data('speed')); // If this element has a Y offset then add it onif ($self.data('offsetY')) {yPos += $self.data('offsetY');}// Put together our final background positionvar coords = '50% '+ yPos + 'px';// Move the background $self.css({ backgroundPosition: coords });// Check for other sprites in this section $('[data-type="sprite"]', $self).each(function() {// Cache the spritevar $sprite = $(this);// Use the same calculation to work out how far to scroll the spritevar yPos = -($window.scrollTop() / $sprite.data('speed')); var coords = $sprite.data('Xposition') + ' ' + (yPos + $sprite.data('offsetY')) + 'px';$sprite.css({ backgroundPosition: coords }); }); // sprites// Check for any Videos that need scrolling$('[data-type="video"]', $self).each(function() {// Cache the videovar $video = $(this);// There's some repetition going on here, so // feel free to tidy this section up. var yPos = -($window.scrollTop() / $video.data('speed')); var coords = (yPos + $video.data('offsetY')) + 'px';$video.css({ top: coords }); }); // video }; // in view }); // window scroll }); // each data-type }); // document ready