<select id="select" name="select[]" multiple>
<option value="1">первое</option>
<option value="2">второе</option>
<optionv value="3">третье</option>
</select>

function getSelection(o){
	if (!o.options) return "";
	var selectedOptions = [];
	for (var i = 0; i < o.options.length; i++) if (o.options[i].selected) selectedOptions.push(o.options[i].value);
	return selectedOptions.join(",");
}