function number_format( number, decimals, dec_point, thousands_sep ) {
    // http://kevin.vanzonneveld.net
    // +   original by: Jonas Raoni Soares Silva (http://www.jsfromhell.com)
    // +   improved by: Kevin van Zonneveld (http://kevin.vanzonneveld.net)
    // +     bugfix by: Michael White (http://crestidg.com)
    // +     bugfix by: Benjamin Lupton
    // +     bugfix by: Allan Jensen (http://www.winternet.no)
    // +    revised by: Jonas Raoni Soares Silva (http://www.jsfromhell.com)    
    // *     example 1: number_format(1234.5678, 2, '.', '');
    // *     returns 1: 1234.57     
 
    var n = number, c = isNaN(decimals = Math.abs(decimals)) ? 2 : decimals;
    var d = dec_point == undefined ? "," : dec_point;
    var t = thousands_sep == undefined ? "." : thousands_sep, s = n < 0 ? "-" : "";
    var i = parseInt(n = Math.abs(+n || 0).toFixed(c)) + "", j = (j = i.length) > 3 ? j % 3 : 0;
    
    return s + (j ? i.substr(0, j) + t : "") + i.substr(j).replace(/(\d{3})(?=\d)/g, "$1" + t) + (c ? d + Math.abs(n - i).toFixed(c).slice(2) : "");
}

http://kevin.vanzonneveld.net

The number_format function from PHP rewritten as javascript. 
From the super badass http://kevin.vanzonneveld.net/techblog/article/phpjs_licensing/

Javascript number formatting  (number_format ported from PHP)

JavaScript