A favorite link of my kids is iknowthat
Each child can be registered, sign in and their own personalized projects are saved. Later they can go back and continue or review. The programs are self-paced and listed by grade level. There is a free version that has ads on the side, or you can purchase a membership. Subjects include: The Arts, Language Arts, Social Studies, Math, Science, Thinking Games and Quiz Central. Check out SoCalVal’s blog for more useful links!