One of our favorite ASP.NET component vendors, Telerik, has recently launched an AJAX developer center portal. Its pretty light weight for the time being and is obviously geared towards Telerik's own ASP.NET rich callback component suite, but does offer some valuable resources for those just getting started with rich client AJAX dev. Resources include: History of AJAX, Getting Started with AJAX, Problems and Challenges with AJAX (ASP.NET specific), etc. Telerik AJAX Developer Center