I very much agree with the Khan Academy comments! The videos have been incredibly helpful for me - not just for math but also for other subjects such as biology, history, etc. The nice part is that the videos aren't just someone lecturing, there are visual aids too. Lately they also have exercises for students to try out. It's all very fun because you get trophies and achievements based on the videos you watch, that sort of thing. Students can easily get motivated!
If you're looking for a resource to simply check if your answers to math problems are correct, I would suggest Wolfram Alpha as well.