C tutorial

Very comprehensive. Not interactive, but programming is just one of those subjects. Explains things very well, and would be a great page to reference or re-explain things from class. http://www.cs.cf.ac.uk/Dave/C/CE.html

C quizzes

http://www.cprogramming.com/quiz/

Great site with quizzes. The rest of the site is fantastic for understanding specific topics in C. FANTASTIC coverage of subject.