I use the Firefox add on "Server Spy". It shows you what web server is serving up the web page that you are currently viewing. Just a little bit of trivia I find interesting. For example www.sun.com shows "Sun-Java-System-Web-Server/7.0", IBM is "IBM_HTTP_Server", and www.apache.org is "Apache/2.2.9 (Unix)" . None of which should be a surprise.
My home page is www.google.com and it has always been served up by "gws". However, today the server has changed and it appears Google is now using IIS. That's Microsoft's web server! Server Spy reports "IIS/Clippy". Earlier today I saw "IIS/Bob".
So what do you think, did Google see the light switch over to Windows and IIS? :-)
\*Update\* It seems that Google has seen the error of their ways and has already upgraded there server and they are now using "Netscape iPlanet". Great choice guys!