Do you know of a search engine that looks in web sites code, not only text? for example to find all the websites that use Jquery I may search for 'jquery-1.3.2.min.js' ....
(I know the question is not directly a programming question but the answer can be very helpful to programmers)
With the exception of <meta> (and a few others), popular search engines (like Google) don't search tags. The reason being is that search engines are looking for content to rank, so any results you find for 'jquery-1.3.2.min.js' will be discussing jQuery rather than if it is being used in the page.
Opera's MAMA (Metadata Analysis and Mining Application) is supposed to do that, but hasn't been made publicly accessible yet. (will be made public in a few months apparently)