Unlike the Apache HTTP server, Microsoft's IIS currently doesn't have a built-in extension offering support for URL rewriting and 'proxying' of server requests. This means that you have to hack up some code to manually process URLs with ASP.NET or implement a nasty 404 handler in classic ASP.
Recently, I needed to centralise a number of services under a single domain name on one of my machines. Normally, an Apache user would use mod_rewrite to proxy requests through to another machine. I wanted to do this on IIS so I evaluated a number of ISAPI filters which purport to offer similar functionality. The candidates were ISAPI Rewrite, IISRewrite and OpUrl. After some spelunking in my Win2K3 virtual machine I decided to purchase an ISAPI Rewrite license. It fairly cheap and questions posted to the support forum are answered promptly.
I posted some setup instructions for proxying requests to a Kerio webmail server on their forums.