Polling 101: A tutorial on the fundamentals of polling from the Roper Center.
Poll & Survey FAQs: from the American Association for Public Opinion Research
20 Questions Journalists Should Ask About Poll Results: An article on how to distinguish between scientific and unscientific polls from Public Agenda.
Only a fraction of polls are published and an even small fraction still is easily attainable. For example, when you in the paper "according to a Gallup Poll published last week" it is not guaranteed that you will be able to find the poll in one of our services. Also, many marketing polls/surveys are prohibitively expensive and therefore in effect not available.