Who's using it: Many Web 2.0 companies and their developers prefer to use Ajax -- including sites and services such as Flickr, Yahoo, the Gap and Amazon's Online Reader. Google developers are way into Ajax; it can be found in Google Maps, Gmail, Google Suggest and Google Groups, among other places.
Why it matters: Ajax creates a faster user experience -- and research has shown faster is almost always better online in consumers' minds. But proponents say the value is more than just speed. Take the Gap, for instance, and its Ajax-powered shopping system. Clothes are shown on the right side of the page where users can use the mouse to select color and size and add an item to the shopping cart -- without ever leaving the page. Shopping-cart contents are listed at the top of the page in a box that pops up whenever a user scrolls across it; again, the user never has to leave the page. Gap chose Ajax for to reduce clicks (and frustrated customers), but the result ends up being a big departure from the traditional online shopping experience of moving from page to page to add items, choose colors and sizes and return to shopping.
Why everyone isn't doing it: Well, they might someday, but the jury is still out on widespread usage because the programming requires a high level of tech knowledge, and still-working-on-it compatibility across systems and operating software. There was quite a stir over lost customers, goodwill and other issues when Gap shut down its site for two weeks last fall while it got the more complex Ajax-powered system up and running.