HowStuffWorks

"HowStuffWorks, a wholly owned subsidiary of Discovery Communications, is the award-winning source of credible, unbiased, and easy-to-understand explanations of how the world actually works. Founded by North Carolina State University Professor Marshall Brain in 1998, the site is now an online resource for millions of people of all ages.
From car engines to search engines, from cell phones to stem cells, and thousands of subjects in between, HowStuffWorks has it covered. No topic is too big or too small for our expert editorial staff to unmask ... or for you to understand. In addition to comprehensive articles, our helpful graphics and informative videos walk you through every topic clearly, simply and objectively. Our premise is simple: Demystify the world and do it in a simple, clear-cut way that anyone can understand. "
HowStuffWorks is a free site that can be used for any class. I've used the site for understanding the digestive system when we were learning about it in physiology.