Here are some examples of some of the best and most popular uses of AJAX:
One of the first major uses of AJAX was the Google Search Autocomplete feature. While you type in your search, a list of searches that you may be about to type pop up that allow you to finish your query without typing the whole word. One of the most useful and popular AJAX interfaces.
Meebo is an online, browser based instant messaging system. It allows you to use almost any IM account or protocol, and is displayed as a set of windows right in the browser. You can click and drag these windows just as if you were using a desktop IM application, and IMs can be sent and received without any reloading of the page.
Lightboxes are the pictures that pop up in the same browser window when clicked to enlarge. They appear as a virtual window right over the content of the website and usually have a nice animation of appearing and sizing to the picture. They often offer gallery support and other nice features. Again, nothing has to be reloaded, and this requires no extra browser windows that add clutter to the user’s experience.
Reddit has a system of voting that allows each user to give every article and up or a down vote, and the total score of the article is displayed right along with the voting mechanism. When a user votes, the choice is highlighted (a good example of a reminder of the dynamic nature of the page) and the vote is counted right on the spot. A user can change his or her vote and vote for as many articles on the page as they want without a single page reload.