ASP.NET web applications include version headers in their default configuration. IIS will also add version headers to the response. For an MVC 4 application running on IIS on Windows 8 you'll get the following headers by default:
Server: Microsoft-IIS/8.0 X-AspNetMvc-Version: 4.0 X-AspNet-Version: 4.0.30319 X-Powered-By: ASP.NET
Version headers have been suppressed throughout this site, but they're enabled at unsecured.nwebsec.com. Go there to see them live.
NWebsec helps you suppress the headers emitted by ASP.NET, see Suppressing version headers on the project website.
Troy Hunt has a nice write-up on version headers on his blog: Shhh… don’t let your response headers talk too loudly. You can also scan for these headers with his online tool ASafaWeb.