We use a variety of online resources, with Khan Academy being a favorite. Originally, before they re-launched the Practice section, we used only the videos. They've done a great job with the problem sets so now it's a mix. As geofizz mentioned the answers are mostly multiple choice so that's a slight negative. However once you get into the upper math topics it's really less of an issue.
When your child gets to pre-algebra, Alcumus from AoPS is a nice supplement. Slightly different approach and the problems are much deeper.
JB
