Here is the list of web application which are using AJAX:
Google provides suggestions as you type and to navigate over the suggestions, the arrow keys are used.
Google Map zooms the location on scrolling the mouse wheel. It just brings the image tiles Asynchronously as you scroll the mouse wheel.
Facebook updates the newsfeed without user intervention that means it makes AJAX calls and brings in the latest feed for the logged in user.
Google Mail is more intuitive than other mail applications. When the subject (link) is clicked, it makes Ajax call and get back the content of the mail and it just render the response as Mail BODY.
As you scroll down the Flickr explore page, you could see the images are being loaded. What happens here is, it just gets the images source (path) Asynchronously from the server.