I've made a page that uses jQuery to allow you to place <div>s on the page based on your mouse coordinates when you click.

The page

And here's the javascript:

$('document').ready(function() {
    $("#canvas").click(function(e){
    var x = e.pageX - this.offsetLeft;
    var y = e.pageY - this.offsetTop;
    $(document.createElement('div')).css({'left':x + 'px', 'top':y + 'px'}).addClass('tile').appendTo('#canvas');
    });
});

I've found that if you mousedown in the div#canvas and mouseup with your pointer over a placed <div> (or vice versa) then a new <div> doesn't get placed. Why is this?