Although this book presents lots of interesting Ajax applications, some with impressive visuals and live data feeds, from a purely practical perspective few of them are as immediately handy as the Validator application. The application has a very basic user interface, but it's the usability that shines here. The help text that appears when invalid data is entered is subtle but compelling. And consider that this help information appears immediately without you having to submit any data—this is the "Ajax" way of doing things, even when an Ajax server request isn't explicitly being made. Of course, the Validator application does involve an Ajax server request for obtaining the city and state for a ZIP code, but the fact remains that the application promotes an "Ajax" feel throughout, even when the validation is taking place solely on the client.