Here’s a little javascript to format numbers like the PHP function “number_format” :

function number_format (number, decimals, dec_point, thousands_sep){
	if (isNaN(number) || number=='') number = 0;

	var decimals 		= decimals != undefined ? decimals : 2,
		dec_point		= dec_point != undefined ? dec_point : '.',
		thousands_sep	= thousands_sep != undefined ? thousands_sep : ' ',
		negative		= number<0;

	if (negative) number *= -1;

	var left 	= parseInt(number, 10),
		right	= Math.round(parseFloat(number.toString().replace(/^d+./, '0.')) * Math.pow(10, decimals));

	left  = left.toString().split('').reverse().join('')
			.match(/d{3}|d{1,2}/g)
			.join(thousands_sep)
			.split('').reverse().join('');

	right = (right / Math.pow(10,decimals)).toString().replace(/^d+./, '').toString();

	if (right.length < decimals) for (var iRight=right.length; iRight < decimals; iRight++) right += '0';

	return (negative?'-':'')+left+dec_point+right;
}

UPDATE : There was a bug in the code. The zeros to the left in the decimals (if there was any) were erased.
UPDATE 2 : There was a second bug, when the number was under 0. Fixed now