Well, Khan works with math, right? And he's not the only one working with self-paced math. Some other folks have mentioned ALEKS (Which my sister and I could not get to work on any of three computers a couple of years ago, and found customer service to be singularly unhelpful).
These programs, and compacting curriculum in general, work very well with math because it is a well-structured field. There is tremendous agreement about the order in which students should learn bits of math. You must learn this before you can learn that. If you have got fractions down, you are ready to learn X, Y, & Z.
Self paced becomes problematic in subjects that are not as well structured.
