Async Pages in ASP.NET 2.0
One of the cool new advanced features in ASP.NET 2.0 is support for pages that execute otherwise blocking operations asynchronously. This allows ASP.NET to re-use the worker thread while waiting for a completion (for example: calling a remote web-service and waiting for the response) and allows the server to execute much more efficiently.
Jeff Prosise now has a great article on MSDN that describes how it works and how you can take advantage of it: http://msdn.microsoft.com/msdnmag/issues/05/10/WickedCode/default.aspx