Until a few months ago, I used to associate the term AJAX with one of the most famous Dutch football teams (we don't call it soccer in Europe). Over the last few month, however, I've kept seeing more and more astonishing web sites, based on standard technologies (HTML, CSS, Javascript) and mostly browser independent... featuring rich client applications! Certainly google adoption of AJAX (use this link for an intro to the technology) has been critical to the spreading of this technology, but there are countless sites featuring it.
Here is a short list of AJAX-based sites (AFAIK):

Over the last couple of weeks I've started playing with AJAX myself. I've found a very rich engine for query processing (also very easy to use) at Ajax Toolbox and I've done some experiments with my newsgroup reading web site. I'll let you know when it is more stable. For now, you can see an example of mine on this blog site, following the link Comments List in the main menu. You'll see a list of talkback messages and can open each of them locally in the page.

I'm pretty sure AJAX and similar technologies will be big. I wonder if it is time for Delphi to resume the Internet Express technologies and turn it into an AJAX engine. I'll certainly do some more experiments with it, having to speak on the topic (AJAX in Delphi) for the European BorCon. Might propose a BOF talk on this topic to the US one, as well.