AJAX - Auto Suggest
To me, the greatest value of AJAX, and the one that makes the web more usable is the use of the "Suggest" pattern.  People don't like to think when their on the web, so the best thing you can do is to help them thing by providing automatic suggestions.  If you like these sites, and would like to implement them in your own site, check out the extreamly simple AJAX Suggest Tutorial.

Google Suggest
Once again, since Google started this first, I'll list it first.  Still probably the best AJAX Suggest I've seen to date, Google Suggest is a beacon to all you aspiring AJAX programmers out there.

ObjectGraph Dictionary
This is a great idea if you don't know how to spell, like me.  It uses the suggest pattern to lookup words from the dictionary for you.  Pretty clever.

This is still one of my favorite uses of AJAX.  This site will help you find the airport code to where you want to go, and lookup flights and list them as their found.

Here is an AJAX suggest site that helps you find song lyrics.  Not bad.