I previously wrote about the Netcraft What's That Site Running? tool (see my post Look up a site's OS and web server), which tells you a site's current and past operating system and web server software. Another site for finding this information is BuiltWith.com. Enter a URL, and BuiltWith lists the server OS information, analytics used, frameworks, document information, encoding, and a list of related sites.
The BuiltWith Internet Technology Trends page lets you view trends and popularity stats for many categories, including ad providers, blog providers, CMS software, and server OS.