Some examples of how we've used Ajax are:
While the administration backend of Wir Sprechen Deutch uses Ajax extensively, we've also used it in the public site. As you click through categories and areas the list of companies and info pages filters down. The same applies to the search - as you type it is filtering results.
This is far more user friendly than 'click -> reload, click ->reload'
Package One Inc is a small site that also didn't want the conventional 'click -> reload, click ->reload' and so we used Ajax to fetch the content of each of the top tabs from the server and display them with a fade in, fade out effect.