What a great idea! The Khan Academy is a teaching site based around videos hosted on youtube and interactive exercises. There are 1800 plus videos, mainly secondary school maths and science at the moment, but Sal Khan hopes to push it out to other areas with time. The exercises are pretty much all maths -except for some on projectile motion.
I have been cracking through the maths exercises trying to find my level -probably around the third year -or year ten in new money. With the school shut for snow I put my 11 year old in front of the site, and she didn't run away screaming. I think that's partly due to the points system, competititive little begger wants a high score.
The maths videos range from simple addition up to moderate calculus so I'd recommend it to anyone looking to brush up their skills or who needs to supplement what their kids are learning in school. Sal is an engaging lecturer, who doesn't force the pace, and he is endearingly bad at using the drawing tools. His explanations are pretty clear and timed well to fit in with the ten minute maximum time for youtube.
There is an exercise map (now in Google Maps -interesting idea), showing how the topics join up, and you are suggested new exercises as you complete old ones. Success here means getting ten answers right in a row, for me this led to me finding my old scientific calculator and checking answers before submitting them, as getting number 9 wrong can lead to the computer hitting the wall. There is also a hints mechanism that works the question through with you -but using it means that you break your streak (or run of right answers), so if you are unsure you may be better off going to the linked video for an explanation.
Probability and Stats for me next I think.