Khan Academy is a free website that can help you learn things from your classes, or teach you brand new things. If you are struggling in a class, or just want to learn something new, check out Khan Academy. From their website:
“Khan Academy offers practice exercises, instructional videos, and a personalized learning dashboard that empower learners to study at their own pace in and outside of the classroom. We tackle math, science, computer programming, history, art history, economics, and more. Our math missions guide learners from kindergarten to calculus using state-of-the-art, adaptive technology that identifies strengths and learning gaps. We've also partnered with institutions like NASA, The Museum of Modern Art, The California Academy of Sciences, and MIT to offer specialized content.”Wolfram Alpha
Wolfram Alpha is a website with computational intelligence that will help you understand your work. With a pro account, Wolfram Alpha will break down the problems step-by-step solution, along with a short description of why they did what they did.