Add comma to numbers every three digits

You could try NumberFormatter.

$(this).format({format:"#,###.00", locale:"us"});

It also supports different locales, including of course US.

Here's a very simplified example of how to use it:

<html>
    <head>
        <script type="text/javascript" src="jquery.js"></script>
        <script type="text/javascript" src="jquery.numberformatter.js"></script>
        <script>
        $(document).ready(function() {
            $(".numbers").each(function() {
                $(this).format({format:"#,###", locale:"us"});
            });
        });
        </script>
    </head>
    <body>
        <div class="numbers">1000</div>
        <div class="numbers">2000000</div>
    </body>
</html>

Output:

1,000
2,000,000

@Paul Creasey had the simplest solution as the regex, but here it is as a simple jQuery plugin:

$.fn.digits = function(){ 
    return this.each(function(){ 
        $(this).text( $(this).text().replace(/(\d)(?=(\d\d\d)+(?!\d))/g, "$1,") ); 
    })
}

You could then use it like this:

$("span.numbers").digits();

You could use Number.toLocaleString():

var number = 1557564534;
document.body.innerHTML = number.toLocaleString();
// 1,557,564,534

Something like this if you're into regex, not sure of the exact syntax for the replace tho!

MyNumberAsString.replace(/(\d)(?=(\d\d\d)+(?!\d))/g, "$1,");