I'm surprised Kahn Academy hasn't been mentioned. It's free and from my experience pretty good. Though I'm not a parent or teacher so I can't speak from either of those viewpoints.
I stand corrected. Can’t edit my original post, though.
Khan hides it well, though - it’s listed smack in the middle of sixth grade math and high school math courses. I skimmed through the list (again) and found some college math but not all. Maybe this is a new offering?
