ASP.NET AJAX can be easily extended to create almost any control you can imagine. Be sure to take a look at the ASP.NET AJAX Control Toolkit.

ASP.NET AJAX SliderExtender Layout and Custom Appearance: Alessandro Gallo describes how to customize the layout and appearance of the ASP.NET AJAX Control Toolkit's SliderExtender control.
Surrounding your Data Table and Dialog Controls with Styled Panels: Matt Berseth describes how to add styled borders to ASP.NET data grid controls using CSS and some of the ASP.NET AJAX Control Toolkit controls.
Yahoo UI Style "Tip of the Day" Dialog using the ASP.NET AJAX PopupControl, DynamicPopulate and DragPanel Extender Controls: Matt Berseth describes how to implement a "Tip of the Day" popup control using ASP.NET AJAX.
"Don't Ask Me Again" Delete Confirm Using a ModalPopup and ASP.NET's Profile Properties: Matt Berseth demonstrates how to to use ASP.NET AJAX to display a ModalPopup prompt to end users and then save the results within the ASP.NET Profile system.
ASP.NET AJAX Extender for Multi-Column Widget Drag & Drop: Omar Al Zabir describes how he built an ASP.NET AJAX Extender that enables drag and drop support for portal-like UI scenarios.
Yahoo UI Style AJAX Progress Indicator: Matt Berseth describes how to implement a YUI style progress panel with ASP.NET AJAX.

Screencast: Introduction to AJAX, ASP.NET AJAX and the AJAX Control Toolkit is a little rough but decent introduction to the world of AJAX on Microsoft ASP.NET.
Use the ASP.NET AJAX CascadingDropDown Control Extender demonstrates how ASP.NET’s standard DropDownList can become AJAX-enabled using the CascadingDropDown control extender from the ASP.NET AJAX Control Toolkit.
Use the ASP.NET AJAX TextBoxWatermark Control Extender demonstrates how to use the TextBoxWatermark control extender from the ASP.NET AJAX Control Toolkit, which adds new watermark properties to all standard TextBox controls.
Use the ASP.NET AJAX Popup Control Extender demonstrates the use of the Popup control extender that ships with the ASP.NET AJAX Control Toolkit.
Use the ASP.NET AJAX AlwaysVisible Control Extender - See how easy it is to anchor a page element to a position on screen, where it always remains visible no matter how far the user may scroll the document.

There are dozens more videos and tutorials on the ASP.NET AJAX documentation site!

AJAX Extenders