Free online services to find what a site is running on

• Netcraft is one of the oldest sites devoted to tracking technology on the Internet. Their service provides a site report that can tell you what web-server a site is running on, host info, uptime etc and also identify related subdomains.
• BuiltWith lists technologies used by a specified site. As they profile thousands of sites, BuiltWith Trends provides consolidated information about the most popular technologies used on the web.
• Snoopy is a bookmarklet for snooping on web pages. Snoopy can 'sniff' out of the page, such as the   doctype, what JS libraries are used in the page, what analytics, what font embedding technique is used, etc. I found it interesting that it also shows the generated source which can be useful to web developers for debugging.