The answer is Mathway. Wikipedia has all the answers for almost every subject in history, and most of them are well thought out and correct. Now take that idea and apply it to math and what do you get? A website that can handle many of types of math problems, can give you the answers to those problems and show all of it's work.
No longer can a teacher just give the homework problem to evaluate this integral:
Because even if the student got the right answer it is possible that he just copied the answer from Mathway. (Click on the equation above to see how to solve the problem.) This means that math will have to start to move away from computations for homework and go towards applications, which is a better way to approach homework in the first place.
I would have loved to have this website when I was going to school.