Textbooks typically have websites for people to use basically for videos, flash cards, exercises, practice quizzes, and practice tests. If your students have textbooks by McDougal Littell, there is a good likelihood that they have access to this website. They include language arts, math, science, social science, and languages for the middle and high school students.
As for the ability to use the textbook websites, it is probably going to be different for each textbook company. As for http://www.classzone.com that I refered to, you can use the self-check quizzes with no need for a code. For anything else like the flash cards for example, the teacher needs to work with the textbook company.