I am a huge fan of edX for exploration of academic and possible majors of interest for my students. They also offer specially designed courses to help students prepare for Advanced Placement (AP®) Exams.
Covering subjects ranging from English language and composition to calculus, biology, statistics and computer science, their prep courses provide access to quality materials at no cost. Teachers can also integrate the course materials to flip the classroom and augment their curriculum if they desire.
Check out the AP prep offerings via edX here.