Update. Sadly the website mentioned in the post is no longer available.
Here is another very interesting Ajax resource well worth a read. This article give some nice examples of things you can do with Ajax, some tutorials and also explains the history of Ajax. The article is several months old and you may already have came across it before – but it certainly deserves a mention here.
Check out DHTMLNirvana’s “Ajax – What is it good for?” (sorry, this link is no longer available).