The Khan Academy is a free site that has lots of great tools for helping people learn. There are thousands (yes,thousands!) of short videos on math (first grade through calculus), physics, biology, chemistry, history, business, and other topic.
Each video is about 10 minutes—long enough to explain one concept, but not so long that it goes past what you're working on to confuse you with the next topic. The videos have lots of diagrams and graphics to help you understand.
There are lots of practice exercises for math. You find out immediately whether your answer was correct. If you're not sure what's going on, you can see the problem solved, step by step.
You can set up a free account to keep track of your progress. You'll earn badges to show what you've mastered.
Teenagers: check out the SAT Math Prep. Mr. Khan works through all the math problems in The Official SAT Study Guide, writing on his computer "blackboard" as he talks. He recommends that you try to take a practice test on your own, score it, and THEN watch him solve the problems you had trouble with.
Teacher Resources (for classroom teachers) are here.
Here are sample videos: