This acronym is mostly used today in the context of Google Maps. I admit that it looks pretty cool - I even posted a while back a technical article about how it uses the AJAX concepts here.
But is this AJAX concept something new? Well, it quickly turns out that it's not. Although we don't see it all over the place, people already used this idea pretty heavily in the past few years. I would pick a notable example: Outlook Web Access.
[update: more AJAX-related posts from Microsofties: here and here]