Смотрите здесь: http://jsfiddle.net/EscRV/
HTML
<select id="mySelect">
<option>value 1</option>
<option value="[new]">new</option>
</select>
JS
// get a cross-browser function for adding events, place this in [global] or somewhere you can access it
var on = (function(){
if (window.addEventListener) {
return function(target, type, listener){
target.addEventListener(type, listener, false);
};
}
else {
return function(object, sEvent, fpNotify){
object.attachEvent("on" + sEvent, fpNotify);
};
}
}());
var mySelect = document.getElementById("mySelect");
on(mySelect, "change", function(){
if (this.value === "[new]") {
var newOptionName = prompt("input new option");
if (newOptionName){
mySelect.options[mySelect.options.length] = new Option(newOptionName);
this.value= newOptionName;
}
}
});