From what I can tell, AJAX (Asynchronous JavaScript and XML) was pioneered by Oddpost, popularized by Google, and named by Jesse James Garrett. I see it as technology that fills the niche between thin client apps and thick client apps. It won’t take over apps that need to be extremely light or require a lot of data transfer, but I can see it becoming popular where the features of both are required.
Information:
Wikipedia
AJAX in Action
AJAX Resources
Dynamic HTML and XML
Blogs:
Adaptive Path
Jesse James Garrett blog
Ajaxian blog
Ben Ramsey
AJAX apps:
Google Mail
Google Maps
Google Suggest
Amazon Loose Diamond finder
Flickr