We have a DS6 who loves Khan Academy. We like it since it helps to assess what he already grasps and focuses in on areas that need more attention. We also like that it isn't just math, but a platform for many other disciplines. So, since he is used to the system for math, it's easy to cross over into history and other subjects. IXL drove us up a wall with it's repetitive tasks that paid no heed to his ability.

We augment Khan academy with some workbooks and other online videos.