How to make div follow scrolling smoothly with jQuery?

Question

In my container there are sections/boxes, but the last one of these boxes should follow scrolling when none of the other boxes are visible.

So, when user scrolls down, he sees a normal sidebar, but when user has went down enough, sidebar ends but the last box starts to follow on the top of the screen. I have seen this a lot on different kind of sites.

My code at the moment:

$(window).scroll(function(){
    $.each($('.follow-scroll'),function(){
        var eloffset = $(this).offset();
        var windowpos = $(window).scrollTop();
        if(windowpos<eloffset.top) {
            var finaldestination = 0;
        } else {
            var finaldestination = windowpos;
        }
        $(this).stop().animate({'top':finaldestination},200);
    });
});

Answer 1

Since this question is getting a lot of views and the tutorial linked in the most voted answer appears to be offline, I took the time to clean up this script.

See it live here: JSFiddle

JavaScript:

(function($) {
    var element = $('.follow-scroll'),
        originalY = element.offset().top;

    // Space between element and top of screen (when scrolling)
    var topMargin = 20;

    // Should probably be set in CSS; but here just for emphasis
    element.css('position', 'relative');

    $(window).on('scroll', function(event) {
        var scrollTop = $(window).scrollTop();

        element.stop(false, false).animate({
            top: scrollTop < originalY
                    ? 0
                    : scrollTop - originalY + topMargin
        }, 300);
    });
})(jQuery);