Ajax Control Toolkit AlwaysVisible Control
The AlwaysVisible control extends any ASP.NET control and lets you pin it to the page so that it appears to float over the background body content when it is scrolled or resized. The extender keeps the position of the control a specified distance from the horizontal and vertical sides.To avoid having the control flash and move when the page loads, you should specify absolute positioning for the control and specify the location.
See the AlwaysVisible Control in action