When teaching a Reporting Services course today, I was told by an attendee that Reporting Services 2008 could be installed without the need for IIS. I read about that, but assumed that it had an internal webserver or something similar that you could activate if you didn’t have IIS (I guess I didn’t really read the documentation, I just made some assumptions). But the attendee pointed me to a webpage that showed how Reporting Services work without IIS. Instead of an internal webserver, Microsoft implemented Reporting Services 2008 as an extension to HTTP.SYS, called an HTTP Listener. That means that Reporting Services gets passed requests for it and IIS never gets those requests. This means that you actually have two processes reacting to requests on port 80. And that’s possible because HTTP.SYS is the process that actually listens on port 80 and hands those requests to either Reporting Services or IIS.

Read more about it here.