To convert your canvas into an image, you can utilize the method canvas.toDataURL().
Subsequently, display this image in a new window using the following code:
var win=window.open();
win.document.write("<img src='"+canvas.toDataURL()+"'/>");
Your users can then perform the standard right-click-save action within that newly opened window.
Below is a functional example:
<!doctype html>
<html>
<head>
<link rel="stylesheet" type="text/css" media="all" href="css/reset.css" /> <!-- resetting css properties -->
<script type="text/javascript" src="http://code.jquery.com/jquery.min.js"></script>
<style>
body{ background-color: ivory; }
canvas{border:1px solid red;}
</style>
<script>
$(function(){
var canvas=document.getElementById("canvas");
var ctx=canvas.getContext("2d");
ctx.fillStyle="gold";
ctx.strokeStyle="blue";
ctx.lineWidth=5;
ctx.rect(50,50,100,100);
ctx.fill();
ctx.stroke();
var win=window.open();
win.document.write("<img src='"+canvas.toDataURL()+"'/>");
}); // end $(function(){});
</script>
</head>
<body>
<canvas id="canvas" width=300 height=300></canvas>
</body>
</html>