One point I would add. The "first real" Ajax application was map.search.ch. map.search.ch is a Swiss site that does similar things to Google Maps, but more. It is an excellent example of how to build a RESTful, dynamic, integrating Ajax site. To help you get started use the following address:
Muelistrasse 3 8143, Stallikon
That is where I used to live. Move the mouse around, and look at the bus stop. Click on it and notice how the site links with sbb.ch (Swiss public transporation). With map.search.ch you can check out how busy parking lots in Zurich are, find phone numbers of marked restaurants, etc. It is very impressive.