Takes a string in format "param1=value1&param2=value2" and returns an javascript object. Improved to handle multiple values with same name and to unescape url encoded values.

jquery.unserialize.js

/**
 * $.unserialize
 *
 * Takes a string in format "param1=value1&param2=value2" and returns an object { param1: 'value1', param2: 'value2' }. If the "param1" ends with "[]" the param is treated as an array.
 *
 * Example:
 *
 * Input:  param1=value1&param2=value2
 * Return: { param1 : value1, param2: value2 }
 *
 * Input:  param1[]=value1&param1[]=value2
 * Return: { param1: [ value1, value2 ] }
 *
 * @todo Support params like "param1[name]=value1" (should return { param1: { name: value1 } })
 * Usage example: console.log($.unserialize("one="+escape("& = ?")+"&two="+escape("value1")+"&two="+escape("value2")+"&three[]="+escape("value1")+"&three[]="+escape("value2")));
 */
(function($){
	$.unserialize = function(serializedString){
		var str = decodeURI(serializedString); 
		var pairs = str.split('&');
		var obj = {}, p, idx;
		for (var i=0, n=pairs.length; i < n; i++) {
			p = pairs[i].split('=');
			idx = p[0]; 
			if (obj[idx] === undefined) {
				obj[idx] = unescape(p[1]);
			}else{
				if (typeof obj[idx] == "string") {
					obj[idx]=[obj[idx]];
				}
				obj[idx].push(unescape(p[1]));
			}
		}
		return obj;
	};
})(jQuery);