Have you ever noticed that IIS tends to brand every HTTP response with the header X-Powered-By: ASP.NET - it will do this even if your site is not powered by ASP.NET!
Here's how to remove it on IIS 7 / 7.5:
- Open Server Manager and select the server name under Internet Information Services (IIS) Manager
- Double click HTTP Response Headers
- Remove the
X-Powered-By
header if it exists

Here's how to remove this header on IIS 6:
- Open up the IIS Management Console
- Right Click on a Web Sites and select Properties
- Click on the HTTP Headers Tab
- Under Custom HTTP Headers click on
X-Powered-By: ASP.NET
and click Remove

Thanks go to Nick Gleason for sending instructions and providing a screenshot for IIS 7