Professionally produced DVD's like chalkdust will contain the complete material for the course, and be completely aligned with the textbook that they are produced for. That may not always be the case with the free stuff like Khan academy. If a parent can fill in what's missing, then the free stuff may be okay.
In my web site,
http://www.mymathspace.net,
I have links to a lot of free math resources, including free technology resources which may help advanced math learners in jr. high/high school dig deeper into math concepts.
As a mathematician and parent, I think it's important for learners to do math in terms of explorations as well as practice. Standard books and videos are good for practice, but for the exploration part - you have to dig around for that.
