service that serves the Web, modified in some way.
One popular application is junk filters such as Junkbuster or Abiprox; what they do is filter out commercial banners and other fluff.
A second is translation: Babelfish and others actually try to do a serious job, but there are also several nonsense translators around.
A third purpose is to provide an augmented service.
One example is a special 'authoring mode' in which authors are provided with information about their pages that end users do not need to see.