This one is a little complex to find at first, so bear with me.
Go to the Higher Ed Page on McGraw Hill's website:
On the LEFT side there is a box labeled "Browse Catalogs". Go there and click either main or specialty and choose a category from the bulleted list that opens.
That will open a list in the main center box on the page. Choose a sub-category. This should open a list right there below the subcategory. The number in parentheses tells how many books are in that subject. Choose a subject from the list. A list of books with cover art pops up. Choose a book by clicking on the title. Now look at the left side, below the "browse catalog" box. For some (but not all) textbooks, there will be an additional box that says "supplements and resources". For some (but not all) textbooks, on the list of supplements and resources is an "Online Learning Center". Click there. (don't click on "Online Learning Center" on the right side of the page. That link takes you to a page teaching teachers how to make OLCs)
The Online Learning Centers vary--some books have AWESOME ones full of useful resources (like the Astronomy one: http://highered.mcgraw-hill.com/sites/0072482621/student_view0/ ). When you click on Online Learning Center, it will take you to a home page for that textbook. On the left side will be a box. In that box, choose "Student Edition". That's where the good stuff is.
Once you click Student Edition, you'll get a mostly blank home page. On the left is a tab, sometimes big and sometimes quite small, with content links in it, including a box that says, "Choose a chapter." Do that. Choose a chapter and check out what's available--quizzes, web links, animations, activities, etc. Very cool and useful stuff. The Astronomy section, for example, includes a chapter summary that, combined with the web links for that chapter, are a complete astronomy course for children. It might not be sufficient for a college course without the complete text book, but it's great for pre-college work.
You have to explore some. Some of the textbooks have GREAT online learning centers, others have interesting ones--an American Music text's OLC includes a listening library with links to songs on iTunes and complete listening guides in Word format--again, not, perhaps, a complete college course in American Music, but more than enough for pre-college and introductory courses.
This is an incredibly valuable free resource for homeschoolers, even if it takes a little exploring to find the good stuff.