Khan Academy started as a series of YouTube videos intended to help Sal Khan's cousins learn math. Now it is shaping into one of the best (and free) tools for classroom math instruction. Teachers at nearly every level of mathematics could make Khan Academy the centerpiece of their curriculum.
Students follow a series of math steps - starting with 1+1 and going as high as their conceptual understanding will allow. Each time a child demonstrated mastery by getting 10 problems in a row correct, they are prompted to move on to a new concept. If they struggle or need help, they can ask for a hint to see a visualization of the problem broken down step by step, watch a video, contact their coach (teacher) or collaborate with peers. Rewards, energy points and the pride associated with self-paced learning will keep students motivated.
Teachers can follow the students as they self-pace through the curriculum and see where they excel or where they need extra assistance. For those of you who love data, there is ample detail on how each child is performing...down to the amount of time they spend on any given problem.
There is MUCH more to Khan Academy, so watch the TED talk and then check out the site to try it out and get set up for yourself.
"I think you just got a glimpse of the future of education." - Bill Gates
Thanks @followmolly for pointing my towards this TED talk.
UPDATE: Remember, that remedial online work can only take you so far. Read about some of the limitations of Khan here.