Reigel's demo page

HTML

<div id="tail"></div>

CSS

#tail {
    position: absolute;
    width: 100%;
    height: 2px;
    background-color: rgba(0, 0, 0, 0.2);
}

JavaScript

$(document).bind('mousemove', function(e){
    $('#tail').css({
       left:  0,
       top:   e.pageY
    });
});