IIS 6.0 with Windows Server 2003 added support for an ISAPI feature called "wildcard mappings".  Wildcard mappings provide a way to configure IIS to cause all requests coming into the server to first be routed to one or more ISAPI extensions for processing.  What is cool about wildcard mappings in IIS 6.0 is that after the ISAPI extension that is processing the wildcard extension is finished, it can then cause IIS to pass control of the request to the extension or internal handler within IIS that normally would process the request.

ASP.NET 2.0 includes built-in support to take advantage of this wildcard mapping feature.  This enables you to run ASP.NET code (or your own custom code) before and after the existing ISAPI extension that processes a non-ASP.NET URL (for example: a .asp, .php or .htm request) executes.

We can use this feature to enable a bunch of cool integration features - including using ASP.NET authentication and authorization features to secure all URLs on a web-server.