AJAX Control Toolkit
ASP.NET Ajax
Ajax is a technology that allows ASP.NET websites to behave much more dynamically. Ajax allows a webpage to update without requiring a full postback. This makes Ajax-enabled sites more responsive, use less bandwidth, and more like desktop applications.
Limiting postbacks to only sections of a webpage opens the doors to a variety of dynamic webpage controls. When you first install the Atlas package (install AJAX), it comes with the basic tools. But that is only the beginning, using those tools one can create all kinds of dynamic controls.
AJAX Control Toolkit
The place to start is definitely with the AJAX Control Toolkit. The Control Toolkit is a collection of AJAX controls that you can simply drag and drop into your existing ASP.NET websites. The list of included controls is incredible, check out the live demo to get a feel for each one of them.
Additionally the package can be installed to support the new Framework 3.5 version. The only other requirement would be, of course, AJAX.
The best part is in the license. These AJAX controls are completely royalty-free. That means you are free to use these controls in your commercial websites and other ASP.NET projects.