Всемогущий,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
</head>
<body>
<canvas width="700" height="500" style="border:1px solid #ccc;margin:10px;cursor:crosshair;" id="canvas"></canvas>
<script>
var canvas = document.getElementById('canvas'),
ctx = canvas.getContext('2d');
canvas.addEventListener('click', function(e){
var x = e.pageX - this.offsetLeft,
y = e.pageY - this.offsetTop;
ctx.fillStyle = '#66FF00';
ctx.beginPath();
ctx.rect(x, y, 15, 15)
ctx.fill();
ctx.stroke();
}, false);
</script>
</body>
</html>