Xmlhttprequest

ajax: a new approach to web applications
Nice whitepaper introducing the ajax model.
guide to using xmlhttprequest
Nice tutorial that does a zip code to city, state lookup live.
sajax toolkit
PHP functions for working with ajax.
using the xml http request object
Nice walkthrough with some specific examples
very dynamic web interfaces
Nice lightweight walkthrough of a live form.
xmlhttprequest & ajax working examples
Umm, examples that work.
xmlhttprequest demo
Three live ajax demos.
xmlhttprequest reference @ xulplanet
The specs in case you want to RTFM.