On my current project I needed to extract info from the http header, in this case send by a custom IE toolbar. However, not all info in the header is available through the request.servervariables.
So how do you get to see what certain pages and toolbars put in the http header? Via Google (what else..) I found ieHTTPHeaders, a nice plugin tool that shows all info of the http header.
Perhaps you find this usefull as well ;)