Sometimes you will need a div to be visible only at the bottom of the view port like a link saying More about us which if the user clicks on it, the page will be scrolled up to let him read more details about the site,
but if the user is already scrolling down to the detailed content, then we need to smoothly hide the More about us link, like what we did in Jidari’s landing page (this scenario is only for resolutions lower than 1024px) like this:

So we made our simple jquery plugin:

jQuery.fn.hideWhileScrollingUp = function (options) {
    var element = this;
    var defaults = {
        speed: 3, // 1 is the slowest
        allowedHeightFromBottom: 200 // after these pixels, it will start to disappear gradually based on the speed
    };
    options = $.extend({}, defaults, options);
    $(window).scroll(function () {
        var elementHeightFromBottom = $(window).scrollTop() + $(window).height() - element.offset().top;
        var delta = elementHeightFromBottom - options.allowedHeightFromBottom;
        var opacity = 1 - (delta / $(window).height() * options.speed);
        element.css("opacity", opacity);
    });
}

and used it like this:

$('#more_about_link').hideWhileScrollingUp();

& it is easy to override its defaults also:

$('#more_about_link').hideWhileScrollingUp({
    speed: 2
});

if you wanna try it online, here is the jsfiddle demo for it.