Domain Details - Displays Server Type, Headers, IP Address, Location Flag, and links to Whois Reports.
Wappalyzer - Wappalyzer is a technology profilers and leads data provider. Create lists of websites and contacts that use certain technologies.
ViewDNS.info - Free DNS related tools including Reverse IP Lookup, DNS Propagation Checker, Chinese Firewall Test...
BuiltWith - Find out the technology behind websites
Whats My DNS - Instantly check your domain names current IP address and DNS record information against multiple name servers located in different parts of the world.
What CMS - WhatCMS.org looks at a variety of factors within a webpage to determine what CMS a website is using.