What web technologies, like programming-languages, frameworks, libraries, ..etc, were used to develop a particular famous web-service, especially Web2.0s.
For example, Ruby on Rails is used to develop: Odeo, A List Apart, Twitter, 43 Things. (Source1) (Source2)
PHP is used to develop: Facebook, Photobucket, Wikipedia, Yahoo, Digg. (Source)
And yes, including Stack Overflow ;)