ajaxpatterns.org
AJAX Patterns is a wiki-based site with all the information you could want regarding AJAX- what it is, what it is used for, how it works, etc. Also available for download are actual 'Ajax Patterns', which are "building blocks of Ajax applications". There is also a list of AJAX-related books...
ajaxian.com
Ajaxian has lots of resources on many topics; not just AJAX. The site has over 350 blog-based articles relating to AJAX development. There are also many great links to other resource sites, and web development / programming related tutorials.
