David Ebbo’s ASP.NET blog : Overriding ASP.NET combine behavior using a VirtualPathProvider
“This article originated from a customer question on the ASP.NET site. What they are trying to achieve is running multiple sites under a single actual ASP.NET application. This can be useful to avoid the overhead of having a different appdomain per site. So the general idea is to have a single application, and to use sub-directories to represent the site. Let’s call them ‘pseudo-sites’ as they are really just directories from the point of view of ASP.NET.”