This is a simple trick that escaped me for a while today. A class that implements IHttpHandler has one main method that looks like this normally:

public void ProcessRequest(HttpContext context) {
    // processing logic
}

Because the HttpContext class is sealed, you can have a hard time mocking it for unit testing purposes. In .net 3.5, Microsoft created a HttpContextBase that newer code uses instead of HttpContext which can be mocked but the IHttpHandler wasn't created to use it.

To get around this, all you really need to do is change your class to the following:

public void ProcessRequest(HttpContext context) {
    ProcessRequest(new HttpContextWrapper(context));
}
public void ProcessRequest(HttpContextBase context) {
    // processing logic
}

When you structure it this way, your unit testing code that passes in a HttpContextBase object will naturally work while still fulfilling the interface requirements.

Like I said, a simple trick but one that took a little while to occur to me.