www.builtwith.com seems to be pretty accurate, probably a good starting point for that information.

Of the four major sites that I've personally worked on, built with was wrong on them all. Two of the four I expected because it's using Javascript to hit a API and I think that would likely be much more difficult to guess correctly. What I didn't expect was that they would all be reported as using php when none of them do. Sure it's a really small sample set and I bet overall they are doing fine but I thought it worth mentioning.

Thanks for the heads up. It explains what I was seeing the other day.