I suspect the "pure awesome" part is at the IHttpModule and IHttpHandler level, where you can do everything except receive and produce grievously invalid HTTP requests. If you need to do that, IIS is extensible via WCF to host more than HTTP.

If you don't use the legacy static accessors like HttpContext.Current, it's actually quite testable. HttpContext.Current is especially bad because it uses thread local storage, so you have to be very careful about when and where you access it.