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 are
var $self = $(this),
offsetCoords = $self.offset(),
topOffset = offsetCoords.top;
// When the window is scrolled...
$(window).scroll(function() {
// If this section is in view
if ( ($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() / $self.data('speed'));
// If this element has a Y offset then add it on
if ($self.data('offsetY')) {
yPos += $self.data('offsetY');
}
// Put together our final background position
var 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 sprite
var $sprite = $(this);
// Use the same calculation to work out how far to scroll the sprite
var yPos = -($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 video
var $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