Kids Discover offers many free worksheets and vocabulary cards that go along with each subject. You type in a unit of study in the search box. For example, "Constitution". When you scroll to the bottom of the page, you will find vocabulary units and teacher's guides you can download.
I especially like the prereading activities that present students with a set of statements about the topic. It asks them to decide whether the statements are true or false before reading. After reading and discussing the topic, students go back and either correct or confirm their previous guesses. It also asks them to provide page numbers that support their answers.
The best part? You don't have to buy the magazines to get the free resources! I find that they are general enough to match up with my units of study without having to buy the magazines, but I think I might buy them in the future. They look like they have some great nonfiction text features kids can sink their teeth into.
Do you use any other magazines for Social Studies or Science? How about prereading activities?