A the Security Summit this week, several people asked me about the .mspx extension Microsoft uses for some of its resources. You can create a custom HTTP handler to process requests for custom extensions. That means you first have to register IIS to pass request for that extension to ASP.NET. This article mentions how to do this. Then, you create a custom handler to process the request, by registering an HTTP handler or handler factory (see more resources on handlers and factories) to do the work. The handler factory's job is to return the right HTTP handler for the request, so ultimately, you are building a handler. The handler might even generate HTML on the fly.

In the case of .mspx extensions, Microsoft uses this extension to generate XML-driven HTML content. This article talks more about the architecture.

http://www.microsoft.com/backstage/bkst_column_46.mspx