Mat Henricson makes a list of 12 perfect cases for AJAX usage on a web page.

I've spent quite some time creating a 2 hour long Ajax course in Swedish. At the end of the course I list 12 cases where I think Ajax is a perfect fit, inspired by Alex Bosworth's 10 Places You Must Use Ajax, which really only has 6 cases. Well, here is my list. I know some of the cases are closely related, and some of them may be out of order when you try them out. Well, at least I'm trying!

I agree with most of the items on the list.  I can see myself using AJAX for suggestiong, validation, and automatic update in many of my Domino application.  I also think that navigation is an area where AJAX is key.  Used correctly, AJAX can be a suitable replacement for Frames.

Even if you think this is something that you already know, the list of examples he has in the list is enough to make it worth the read.