Ajax is everywhere. Leave a post on the WordPress Support Forum and you will see your comment get highlighted after you save it, without the page appearing to reload. If you are using a WordPressMU based blog like WordPress.com, then you are seeing a ton of Ajax boxes and features from the Write Post panel to the Feedback button window.
Ajax is a new web technology that allows users to send and receive data to and from a server without requiring the browser to refresh or reload. Expect to see Ajax pop up everywhere in just about everything you do.
Are you currently using Ajax enhanced or driven utilities, tools, or software? What do you think of it? Is it really useful? What kinds of things that you currently use might benefit from an Ajax interface? I want to know.