One website that I just discovered is No Red Ink. This site is free and enables teachers to register and sign up entire classes. Teachers can assign practice assignments and quizzes to each class, or students can learn at their own pace.
Students fill in a brief interest survey, which then affects the content in each question. I stated that I like the Harry Potter books, so some of the character names appeared in my question.
Give No Red Ink a try and see what you think!