There's builtwith.com - looks at server response headers etc. Doesn't always detect everything because hiding such information is a security advantage for website owners.
Hopefully not. For the most part, that list was compiled from either personal experience or networking (E.g., sitting in on a "how we use Python at Quora" discussion at Pycon, for example). Unless one specifically crafts their HTML to say "Powered by Python" or "Powered by Rails" (sometimes people do, but even those aren't necessarily trustworthy), neither Rails nor Django projects are likely to contain any artifacts that would indicate what they were built with.
