<p>How do you unit test a <code>HttpModule</code> in asp.net given that <code>HttpApplication</code> and <code>HttpContext</code> do no implement an interface ?</p>

In the past before moving to ASP.NET MVC, I used
this library
Phil Haack created for Unit Testing anything that used the HttpApplication and HttpContext.  It in turned used a
Duck Typing library
.
Unfortunately, this was the best way to do it.  ASP.NET was not made to be easily testable.  When they worked on ASP.NET MVC, one of the goals is to get rid of these headaches by making the framework more testable.