var Draggable = function(element) {
    element.addEventListener('mousedown', Draggable.mousedown, false);
    document.addEventListener('mouseup', Draggable.mouseup, false);
}
Draggable.mousedown = function(e) {
    e.target.style.position = 'relative';
    document.addEventListener('mousemove', Draggable.mousemove, false);

    var startingPosition = Draggable.mouseposition(e);
    var targetPostion = Draggable.targetposition(e);

    Draggable.offset = {
        x: targetPostion.x - startingPosition.x,
        y: targetPostion.y - startingPosition.y,
    }
    Draggable.dragging = e.target;
}
Draggable.mousemove = function(e) {
    var mouseposition = Draggable.mouseposition(e);
    var moveposition = {
        x: mouseposition.x + Draggable.offset.x,
        y: mouseposition.y + Draggable.offset.y
    }

    Draggable.dragging.style.left = moveposition.x + 'px';
    Draggable.dragging.style.top = moveposition.y + 'px';
}
Draggable.mouseup = function(e) {
    document.removeEventListener('mousemove', Draggable.mousemove, false);
}
Draggable.targetposition = function(e) {
    var extractInt = function(value) {
        var n = parseInt(value);
        return n == null || isNaN(n) ? 0 : n;
    }

    return {
        x: extractInt(e.target.style.left),
        y: extractInt(e.target.style.top),
    }
}
Draggable.mouseposition = function(e) {
    var posx = 0;
    var posy = 0;
    if (!e) var e = window.event;
    if (e.pageX || e.pageY) {
        posx = e.pageX;
        posy = e.pageY;
    }
    else if (e.clientX || e.clientY) {
        posx = e.clientX + document.body.scrollLeft + document.documentElement.scrollLeft;
        posy = e.clientY + document.body.scrollTop + document.documentElement.scrollTop;
    }
    return {
        x: posx,
        y: posy
    };
}

var draggableDiv = document.getElementById("dragDiv");
var anotherDraggableDiv = document.getElementById("dragAnotherDiv");
Draggable(draggableDiv);
Draggable(anotherDraggableDiv);​