I have not used this one myself, but a bunch of folks I respect really like the FREE
My son used http://www.kahnacademy.com to help with enough Trig to do Physics. This is not specifically Christian, but you can find things K-college. I would monitor.
My daughter paid her own way to anatomy camp with Landry Academy. We haven’t taken any of their (paid) online classes, but I think they’d be good! www.landryacademy.com
What ones have your used? Why do you like them?