Builtwith.com : Find out what a website's frontend is built with
This is a very interesting website which allows you to understand the technology behind the websites you visit. Here is more from its about page
BuiltWith is a web site profiler tool. Upon looking up a page, BuiltWith returns all the technologies it can find on the page. BuiltWithÃ¢â‚¬â„¢s goal is to help developers, researchers and designers find out what technologies pages are using which may help them to decide what technologies to implement themselves.
BuiltWith technology tracking includes widgets (snap preview), analytics (Google, Nielsen), frameworks (.NET, Java), publishing (WordPress, Blogger), advertising (DoubleClick, AdSense), CDNs (Amazon S3, Limelight), standards (XHTML,RSS), hosting software (Apache, IIS, CentOS, Debian) .