AJAX : The Official Microsoft ASP.NET Site

ASP.NET AJAX is the free Microsoft AJAX framework for building highly interactive and responsive web applications that work across all popular browsers. The ASP.NET AJAX framework includes Server-Side ASP.NET AJAX, Client-Side ASP.NET AJAX, the AJAX Control Toolkit, and the jQuery library. ASP.NET AJAX enables developers to choose their preferred method of AJAX development, whether it is server-side programming, client-side programming, or a combination of both. [Detail]